Trunk Somites Research Articles

Redefining the head–trunk interface for the neural crest

The head–trunk interface lies at the occipito-cervical boundary, which corresponds to the somite 5/6 level. Previous studies have demonstrated that neural crest cells also behave differently either side of this boundary and that this may be due to intrinsic differences between cranial and trunk crest. However, it is also possible that some of the observed differences between cranial and trunk crest are assigned by environmental cues. We have therefore scrutinised the behaviour of the neural crest cells generated either side of the occipito-cervical boundary in chick and, interestingly, find that both behave in a truncal fashion by traversing the anterior half of their adjacent somites. Furthermore, although not previously described, we find that transient DRGs form opposite somites 4 and 5. Crest cells produced anterior of the somite 3/4 boundary avoid the somites and behave in a non-truncal fashion; these cells populate the pharyngeal arches, and thus contribute to the developing head. We have further shown, via somite transplantations, that differential behaviour of the posterior versus anterior occipital crest is assigned by the somites. If somites 1 to 3 are replaced by trunk somites, then the anterior occipital crest will behave in a truncal fashion by invading the somites. Correspondingly, if these anterior occipital somites are transplanted in place of trunk somites, they perturb the migration of trunk crest. Thus, for the neural crest, the head–trunk interface does not lie at the occipito-cervical boundary, but rather lies at the somite 3/4 level and is defined by the somites. The fact that this boundary lies at the somite 3/4 level in chick is significant as it reflects the more ancient posterior occipital boundary; in fish, only the first three somites contribute to the occipital bone.

Disruption of Segmental Neural Crest Migration and Ephrin Expression in Delta-1 Null Mice

Neural crest cells migrate segmentally through the rostral half of each trunk somite due to inhibitory influences of ephrins and other molecules present in the caudal-half of somites. To examine the potential role of Notch/Delta signaling in establishing the segmental distribution of ephrins, we examined neural crest migration and ephrin expression in Delta-1 mutant mice. Using Sox-10 as a marker, we noted that neural crest cells moved through both rostral and caudal halves of the somites in mutants, consistent with the finding that ephrinB2 levels are significantly reduced in the caudal-half somites. Later, mutant embryos had aberrantly fused and/or reduced dorsal root and sympathetic ganglia, with a marked diminution in peripheral glia. These results show that Delta-1 is essential for proper migration and differentiation of neural crest cells. Interestingly, absence of Delta-1 leads to diminution of both neurons and glia in peripheral ganglia, suggesting a general depletion of the ganglion precursor pool in mutant mice.

New arthropods from the Lower Devonian Hunsru¨ck Slate (Lower Emsian, Rhenish Massif, western Germany)

Four new genera and species of arthropod, Cambronatus brasseliWingertshellicus backesiEschenbachiellus wuttkensis and Magnoculus blindi, are described from the Hunsru¨ck Slate (Lower Emsian) of Germany. All four occur in the Wingertshell Member in the vicinity of Bundenbach. They preserve remarkable details of the ventral morphology, including the appendages, as a result of pyritization. In each case the body consists of just two tagmata, a cephalon and a large number of similar trunk somites. Both CambronatusWingertshellicus have fluke‐like appendages making up a tail fan. The affinities of these arthropods lie with the Crustacea, apart from Magnoculus, which is an arachnomorph, but they do not fall within those clades with modern representatives. They show that morphologies other than those represented by trilobites and modern arthropod groups persisted long after the Cambrian, at least in muddy bottom marine settings.

Signals from the Yolk Cell Induce Mesoderm, Neuroectoderm, the Trunk Organizer, and the Notochord in Zebrafish

We have analyzed the role of the zebrafish yolk cell in the processes of mesoderm induction and establishment of the organizer. By recombining blastomere-free yolk cells and animal cap tissue we have shown that the yolk cell itself can induce mesoderm in neighboring blastomeres. We further demonstrate the competence of all blastomeres to form mesoderm, suggesting the endogenous mesoderm inducing signal to be locally restricted. Ablation of the vegetal third of the yolk cell during the first 20 min of development does not interfere with mesoderm formation in general, but results in completely ventralized embryos. These embryos lack the notochord, neuroectoderm, and the anterior-most 14–15 somites, demonstrating that the ablation affects the formation of the trunk-, but not the tail region of the embryo. This suggests the presence of a trunk organizer in fish. The dorsalized mutant swirl (zbmp-2b) shows expanded dorsal structures and missing ventral structures. In contrast to the phenotypes obtained upon the ablation treatment in wild-type embryos, removal of the vegetal-most yolk in swirl mutants results in embryos which do form neuroectoderm and anterior trunk somites. However, both wild-type and swirl mutants lack a notochord upon vegetal yolk removal. These ablation experiments in wild-type and swirl mutant embryos demonstrate that in zebrafish dorsal determining factors originate from the vegetal part of the yolk cell. These factors set up two independent activities: one induces the notochord and the other is involved in the formation of the neuroectoderm and the trunk region by counteracting the function of swirl. In addition, these experiments show that the establishment of the anteroposterior axis is independent of the dorsoventral axis.

A molecular mechanism enabling continuous embryonic muscle growth - a balance between proliferation and differentiation.

Embryonic muscle growth requires a fine balance between proliferation and differentiation. In this study we have investigated how this balance is achieved during chick development. Removal of ectoderm from trunk somites results in the down-regulation of Pax-3 expression and cell division of myogenic precursors is halted. This initially leads to an up-regulation of MyoD expression and to a burst in terminal differentiation but further muscle growth is arrested. Locally applied bone morphogenetic protein-4 (BMP-4) to somites mimics the effect of the ectoderm and stimulates Pax-3 expression which eventually results in excessive muscle growth in somites. Surprisingly, BMP-4 up-regulates expression of noggin which encodes a BMP-4 antagonist. This suggests that the proliferation enhancing activity of BMP-4 can be limited via up-regulation of noggin and that myogenic cells differentiate, as an intrinsic property, when deprived of BMP-4 influence. In contrast to BMP-4, Sonic hedgehog (Shh) locally applied to somites arrests muscle growth by down-regulation of Pax-3 and immediate up-regulation of MyoD expression. Such premature muscle differentiation in somites at tongue and limb levels prevents myogenic migration and thus tongue and limb muscle are not formed. Therefore, precise limitation of differentiation, executed by proliferative and Pax-3 promoting signals, is indispensable for continuous embryonic muscle growth.

Cell Death in the Avian Sclerotome

In this study the occurrence of apoptotic cells in chick embryo trunk somites, between 2.5 and 4 days of development, has been examined using anin situnick-end-labeling method (TUNEL) to identify nuclei in which DNA is undergoing fragmentation. At 2.5 days of development, apoptotic cells were found in the sclerotome with a distribution that depended on the rostrocaudal level in the trunk. At the most rostral levels (somites 1–18), dying cells were present primarily in the rostral half of the ventral sclerotome; at midlevels (somites 19–26), they were present throughout the ventral sclerotome; and at caudal levels (somites 27–32), no dying cells were present. By 4 days of development, the number of dying cells in the sclerotome was sharply reduced, and those present were primarily distributed to the caudal side of the intrasclerotomal fissure. Double labeling of cells for both TUNEL and the HNK-1 epitope, at 2.5 days, indicated that the majority of the dying cells were not neural crest cells. Further, dying cells in the rostral somite half were present largely in regions of the sclerotome that labeled poorly for HNK-1. It was confirmed that apoptotic neural crest cells retain the HNK-1 epitope and therefore would have been observed if present. Neural crest cells only appeared to be apoptotic in relatively small numbers and only at the ventral border of the sclerotome. Examination of DiI-labeled neural crest cells confirmed that the dying cells in the body of the somite were not primarily neural crest cells. Two hypotheses regarding the TUNEL-positive cells in the sclerotome were experimentally tested. First, that they originate from the somitocoel compartment of the somite, because their distribution patterns at 4 days were similar to those of somitocoel cells. To test this, somitocoel cells were labeled with carboxyfluorescein and grafted into host embryosin ovo. Results showed that these cells did not become apoptotic and that the dying cells were therefore not derived from the somitocoel. Second, the hypothesis was tested that the distribution patterns of the dying cells in the sclerotome are determined by factors outside the somite itself. Somites and segmental plates were transplanted into hostsin ovowith reversed orientation, after which the patterns of dying cells were examined using nile blue sulfate staining. The results indicated that the patterns were unchanged after a further 2 days incubation, suggesting that the patterns of cell death in the sclerotome are not determined solely from within the somite. The distribution of the cell death-associated gene products, bcl-2, bax, and interleukin-1β converting enzyme, indicates that although these proteins are segmentally distributed in the dermomyotome and in the rostrodorsal quadrant of the sclerotome, their patterns are not directly correlated with the distribution of dying cells.

Axial Mesoderm and Patterning of the Zebrafish Embryo

SYNOPSIS. It has long been recognized that the notochord, a derivative of the gastrula organizer, is important for patterning the vertebrate embryo. Experimental manipulations of amphibian and chick embryos demonstrate the notochord's influence on neural tube and somite differentiation. Recent studies implicate the notochord-derived signal Sonic hedgehog. In zebrafish, genetic strategies have enabled the recovery of mutations that disrupt notochord development. Embryos mutant for floating head or no tail lack differentiated notochord and, consistent with encoding transcription factors, both genes behave cell autonomously in genetic mosaics: wild-type cells develop into notochord in mutant embryos, whereas mutant cells fail to contribute to wild-type notochord. In floating head mutants, axial mesoderm fated to form notochord respecines to paraxial mesoderm, differentiating into muscle instead. As a result, fused somites develop across the trunk midline. In no tail mutants, mesenchymal cells related by lineage to notochord are found in the midline, and although trunk somites are bilaterally paired, they lack Engrailed-expressing muscle pioneer cells. In the presence of wild-type notochord, however, no tail mutant cells can differentiate muscle pioneers, suggesting that induction of muscle pioneers is a normal function of notochord. Notochord is also involved in induction of floor plate at the ventral midline of the vertebrate neural tube; yet in floating head and no tail mutants, floor plate cells develop in the absence of differentiating notochord. Expression of sonic hedgehog by newly-forming axial mesoderm at gastrulation may account for floor plate induction prior to notochord differentiation. Thus, analyses of zebrafish notochordless mutants suggest a role for both early and late signaling by cells of the notochord lineage during embryogenesis.

The taphonomy and affinities of the problematic fossilMyoscolexfrom the Lower Cambrian Emu Bay Shale of South Australia

Most of the specimens ofMyoscolex atelesGlaessner, 1979, the most abundant soft-bodied taxon in the Big Gully fauna from the Lower Cambrian Emu Bay Shale of South Australia, preserveonlythe phosphatized trunk muscles, in striking contrast to the organic residues that characterize other Burgess-Shale-type biotas. This is the oldest phosphatized muscle tissue and the first thus far reported from the Cambrian. The extent of phosphatization implies a source in addition to the animal itself, and this is reflected in high levels of phosphate in the Big Gully sequence compared to other shales. The apparent anomaly posed by the extensive preservation of labile muscle tissue as opposed to the more decay resistant cuticle is explained by the role of bacterial processes in the preservation of soft tissues. New specimens ofMyoscolexreveal a variable number of trunk somites with possible tergites, and flap-like appendages. There is evidence for at least three eyes on the head, and a proboscis may have been present. An annelid affinity is rejected andMyoscolexis reinterpreted as anOpabinia-likeanimal with possible affinities with the arachnomorph arthropods.

Mutations affecting the formation of the notochord in the zebrafish, Danio rerio.

In a large scale screen for mutants with defects in the embryonic development of the zebrafish we identified mutations in four genes,floating head (flh), momo (mom), no tail (ntl), and doc, that are required for early notochord formation. Mutations in flh and ntl have been described previously, while mom and doc are newly identified genes. Mutant mom embryos lack a notochord in the trunk, and trunk somites from the right and left side of the embryo fuse underneath the neural tube. In this respect mom appears similar to flh. In contrast, notochord precursor cells are present in both ntl and doc embryos. In order to gain a greater understanding of the phenotypes, we have analysed the expression of several axial mesoderm markers in mutant embryos of all four genes. In flh and mom, Ntl expression is normal in the germ ring and tailbud, while the expression of Ntl and other notochord markers in the axial mesodermal region is disrupted. Ntl expression is normal in doc embryos until early somitic stages, when there is a reduction in expression which is first seen in anterior regions of the embryo. This suggests a function for doc in the maintenance of ntl expression. Other notochord markers such as twist, sonic hedgehog and axial are not expressed in the axial mesoderm of ntl embryos, their expression parallels the expression of ntl in the axial mesoderm of mutant doc, flh and mom embryos, indicating that ntl is required for the expression of these markers. The role of doc in the expression of the notochord markers appears indirect via ntl. Floor plate formation is disrupted in most regions in flh and mom mutant embryos but is present in mutant ntl and doc embryos. In mutant embryos with strong ntl alleles the band of cells expressing floor plate markers is broadened. A similar broadening is also observed in the axial mesoderm underlying the floor plate of ntl embryos, suggesting a direct involvement of the notochord precursor cells in floor plate induction. Mutations in all of these four genes result in embryos lacking a horizontal myoseptum and muscle pioneer cells, both of which are thought to be induced by the notochord. These somite defects can be traced back to an impairment of the specification of the adaxial cells during early stages of development. Transplantation of wild-type cells into mutant doc embryos reveals that wild-type notochord cells are sufficient to induce horizontal myoseptum formation in the flanking mutant tissue. Thus doc, like flh and ntl, acts cell autonomously in the notochord. In addition to the four mutants with defects in early notochord formation, we have isolated 84 mutants, defining at least 15 genes, with defects in later stages of notochord development. These are listed in an appendix to this study.

Somite subdomains, muscle cell origins, and the four muscle regulatory factor proteins.

We show by immunohistology that distinct expression patterns of the four muscle regulatory factor (MRF) proteins identify subdomains of mouse somites. Myf-5 and MyoD are, at specific stages, each expressed in both myotome and dermatome cells. Myf-5 expression is initially restricted to dorsal cells in all somites, as is MyoD expression in neck somites. In trunk somites, however, MyoD is initially expressed in ventral cells. Myogenin and MRF4 are restricted to myotome cells, though the MRF4-expressing cells are initially less widely distributed than the myogenin-expressing cells, which are at all stages found throughout the myotome. All somitic myocytes express one or more MRFs. The transiently distinct expression patterns of the four MRF proteins identify dorsal and ventral subdomains of somites, and suggest that skeletal muscle cells in somites originate at multiple sites and via multiple molecular pathways.

The ventral and posterior expression of the zebrafish homeobox gene eve1 is perturbed in dorsalized and mutant embryos

We have identified and characterized zebrafish eve1, a novel member of the Drosophila even-skipped (eve) gene family. eve1 RNAs are expressed initially in late blastulae with a peak during the gastrula stage, at which time expression is confined to ventral and lateral cells of the marginal zone of the zebrafish embryo. Later, eve1 transcripts are located in the most posterior part of the extending tail tip. We show that LiCl, known to dorsalize Xenopus embryos, has the same effect in zebrafish, resulting in embryos with exaggerated dorsoanterior structures. In LiCl-treated embryos, eve1 transcripts are completely absent. eve1 is therefore a marker of ventral and posterior cells. In the light of its ventroposterior expression domain, the localization of eve1 transcripts was analysed in spadetail (spt) and no tail (ntl), two mutants with abnormal caudal development. In sptb140 homozygous mutants, there is an accumulation of cells in the tail region, resulting from inadequate migratory behaviour of precursors to the trunk somites. These cells, in their abnormal environment, express eve1, emphasizing the correlation between ventroposterior position and eve1 expression. In homozygous mutant embryos for the gene ntl (the homologue of mouse Brachyury, originally called Zf-T), posterior structures are missing (M. E. Halpern, C. B. Kimmel, R. K. Ho and C. Walker, 1993; Cell In press). While mutant and wild-type embryos do not differ in their eve1 transcript distribution during gastrulation, eve1 expression is absent in the caudal region of mutant ntl embryos during early somitogenesis, indicating a requirement for ntl in the maintenance of eve1 expression during tail extension. Our findings suggest that eve1 expression is correlated with a ventral and posterior cell fate, and provide first insights into its regulation.

The Tantulocaridan Life Cycle: the Circle Closed?

ABSTRACT The discovery of a new stage in the life cycle of the Tantulocarida is reported. A sexual female, collected from a deep-sea harpacticoid copepod host, was removed from the trunk sac of the preceding tantulus larva. This female is a free-living and nonfeeding stage which presumably mates with the free-swimming adult male previously described. The female comprises a cephalothorax, probably incorporating 2 limbless thoracic somites, 2 free pedigerous trunk somites, and 3 limbless trunk somites. It also possesses paired antennules, the only well-defined cephalic appendages present at any stage in tantulocaridan life history. There is a median genital aperture, the copulatory pore, located ventrally on the cephalothorax at about the level of the incorporated first thoracic somite. This is interpreted as further evidence of a sister-group relationship between the Tantulocarida and the Thecostraca. The known life-cycle stages of the Tantulocarida are now interpreted as forming two cycles, one sexual, the other parthenogenetic.

The developmental fate of the cephalic mesoderm in quail-chick chimeras

The developmental fate of the cephalic paraxial and prechordal mesoderm at the late neurula stage (3-somite) in the avian embryo has been investigated by using the isotopic, isochronic substitution technique between quail and chick embryos. The territories involved in the operation were especially tiny and the size of the transplants was of about 150 by 50 to 60 microns. At that stage, the neural crest cells have not yet started migrating and the fate of mesodermal cells exclusively was under scrutiny. The prechordal mesoderm was found to give rise to the following ocular muscles: musculus rectus ventralis and medialis and musculus oblicus ventralis. The paraxial mesoderm was separated in two longitudinal bands: one median, lying upon the cephalic vesicles (median paraxial mesoderm--MPM); one lateral, lying upon the foregut (lateral paraxial mesoderm--LPM). The former yields the three other ocular muscles, contributes to mesencephalic meninges and has essentially skeletogenic potencies. It contributes to the corpus sphenoid bone, the orbitosphenoid bone and the otic capsules; the rest of the facial skeleton is of neural crest origin. At 3-somite stage, MPM is represented by a few cells only. The LPM is more abundant at that stage and has essentially myogenic potencies with also some contribution to connective tissue. However, most of the connective cells associated with the facial and hypobranchial muscles are of neural crest origin. The more important result of this work was to show that the cephalic mesoderm does not form dermis. This function is taken over by neural crest cells, which form both the skeleton and dermis of the face. If one draws a parallel between the so-called "somitomeres" of the head and the trunk somites, it appears that skeletogenic potencies are reduced in the former, which in contrast have kept their myogenic capacities, whilst the formation of skeleton and dermis has been essentially taken over by the neural crest in the course of evolution of the vertebrate head.

NEW TANTULOCARID, STYGOTANTULUS STOCKI, PARASITIC ON HARPACTICOID COPEPODS, WITH AN ANALYSIS OF THE PHYLOGENETIC RELATIONSHIPS WITHIN THE MAXILLOPODA

ABSTRACT A new genus of Tantulocarida, Stygotantulus, is described based on material from an anchialine pool on Lanzarote, Canary Islands. It is the most primitive tantulocarid known and is ectoparasitic on representatives of at least two families of harpacticoid copepods. It is distinguished by the presence of 7 abdominal somites in the tantulus larva. The musculature of the penis on trunk somite 7 of the male suggests that it is derived by modification of the seventh thoracopods. The importance of trunk somite numbers in maxillopodan systematics is reexamined and an attempt is made to apply the concept of homology to the developmental processes determining somite numbers. The classification of the Crustacea, including the Tantulocarida, proposed by Starobogatov (1986), is criticized and the validity of the developmental-functional concept of the prototagma, as used by Starobogatov, is refuted. A new scheme of phylogenetic relationships among seven major maxillopodan groups is presented.

Combined Vital Dye Labelling and Catecholamine Histofluorescence of Transplanted Ciliary Ganglion Cells

We have utilized the carbocyanine dye, DiI, to label suspensions of dissociated ciliary ganglion cells removed from 6 to 12 day old quail embryos. Some of the cells were injected into the trunk somites of 2.5 - 3 day old chick embryos along pathways where neural crest cells migrate to form sensory and sympathetic ganglia, aortic plexuses and the adrenal medulla; the remainder of the cells were cultured to check their viability and the persistence of the DiI label. Embryos were incubated for 1 – 8 days post-injection, fixed in 4% paraformaldehyde/0.25% glutaraldehyde and processed for cryostat sectioning. DiI-labelled cells were readily identifiable in culture and in sections of embryos at all stages examined. Several cell types were identified, based on their morphology and soma size. These included cells with large cell bodies and bright DiI-labelling that appeared to be neurons and smaller, more weakly labelled cells that appeared non-neuronal. The latter presumably had divided several times, accounting for their reduced levels of dye. Many of the DiI-labelled cells were found in and around neural crest-derived sympathetic ganglia, aortic plexuses and adrenomedullary cords, but were rarely observed in dorsal root ganglia. The aldehyde fixative (Faglu mixture) used in this study reacts with catecholamines to form a bright reaction product in adrenergic cells including those in the sympathetic ganglia and the adrenal medulla. The catecholamine biproduct and the DiI in the same cell can easily be viewed with different fluorescent filter sets. A variable number of the DiI-labelled cells in these adrenergic sites contained catecholamines. Cells derived from younger 6 day ciliary ganglion dissociates exhibited detectable catecholamine neurotransmitters earlier and more frequently than those derived from 8 day embryos. The presence of cells exhibiting both bright DiI and catecholamine fluorescence is consistent with previous indications that post-mitotic ciliary ganglion neurons can undergo phenotypic conversion from cholinergic to adrenergic when transplanted to the trunk environment.

Interactions and fates of avian craniofacial mesenchyme.

Craniofacial mesenchyme is composed of three mesodermal populations - prechordal plate, lateral mesoderm and paraxial mesoderm, which includes the segmented occipital somites and the incompletely segmented somitomeres - and the neural crest. This paper outlines the fates of each of these, as determined using quail-chick chimaeras, and presents similarities and differences between these cephalic populations and their counterparts in the trunk. Prechordal and paraxial mesodermal populations are the sources of all voluntary muscles of the head. The latter also provides most of the connective precursors of the calvaria, occipital, otic-parietal and basisphenoid tissues. Lateral mesoderm is the source of peripharyngeal connective tissues; the most rostral skeletal tissues it forms are the laryngeal and tracheal cartilages. When migrating neural crest cells encounter segmented paraxial mesoderm (occipital and trunk somites), most move into the region between the dermamyotome and sclerotome in the cranial half of each somite. In contrast, most cephalic crest cells migrate superficial to somitomeres. There is, however, a small subpopulation of the head crest that invades somitomeric mesoderm. These cells subsequently segregate presumptive myogenic precursors of visceral arch voluntary muscles from underlying mesenchyme. In the neurula-stage avian embryo, all paraxial and lateral mesodermal populations contain precursors of vascular endothelial cells, which can be detected in chimaeric embryos using anti-quail endothelial anti-bodies. Some of these angioblasts differentiate in situ, contributing directly to pre-existing vessels or forming isolated, nonpatent, cords that subsequently vesiculate and fuse with nearby vessels. Many angioblasts migrate in all directions, invading embryonic mesenchymal and epithelial tissues and participating in new blood vessel formation in distant sites. The interactions leading to proper spatial patterning of craniofacial skeletal, muscular, vascular and peripheral neural tissues has been studied by performing heterotopic transplants of each of these mesodermal and neural crest populations. The results consistently indicate that connective tissue precursors, regardless of their origin, contain spatial information used by the precursors of muscles and blood vessels and by outgrowing peripheral nerves. Some of these connective tissue precursors (e.g. the neural crest, paraxial mesoderm) acquire their spatial programming while in association with the central nervous system or developing sensory epithelia (e.g. otic, optic, nasal epithelia).

The development of acetylcholinesterase activity in the embryonic nervous system of the frog, Xenopus laevis

Histochemical detection of acetylcholinesterase (AChE) activity in Xenopus embryos was found first in primary motoneurons, Rohon-Beard neurons and somitic myotubes at early tail bud stages. At late tail bud stages all primary neurons, including primary interneurons, cranial ganglion cells and ventral brainstem cells expressed this enzyme. The onset of detectable AChE activity in some primary neurons occurred near the time of initial axon outgrowth, whereas in others it occurred at much later stages. At early independent-feeding and continuous-swimming stages nearly all seemingly postmitotic neurons began to express AChE activity, and by the beginning of limb bud stages, when many secondary neuronal populations were going through their final rounds of mitosis, nearly all CNS cells outside the ventricular zone were intensely stained. Thus, the onset of detectable AChE activity in secondary neurons occurred near the time of their final mitoses. In trunk somites the enzyme activity initially was located diffusely throughout the myotube, and with progressing development it became localized to the myocommata. From the onset of AChE activity both head somites and head muscles had discrete patches of reaction product all over their surfaces. The onset of detectable AChE activity occurred in muscles near the time that they were contacted by motor axons. These data demonstrate that the primary neurons are the first to express AChE activity, and that as the secondary neurons begin to proliferate, AChE is expressed by nearly all embryonic neuronal populations.

The differing effects of occipital and trunk somites on neural development in the chick embryo.

In all higher vertebrate embryos the sensory ganglia of the trunk develop adjacent to the neural tube, in the cranial halves of the somite-derived sclerotomes. It has been known for many years that ganglia do not develop in the most cranial (occipital) sclerotomes, caudal to the first somite. Here we have investigated whether this is due to craniocaudal variation in the neural tube or crest, or to an unusual property of the sclerotomes at occipital levels. Using the monoclonal antibody HNK-1 as a marker for neural crest cells in the chick embryo, we find that the crest does enter the cranial halves of the occipital sclerotomes. Furthermore, staining with zinc iodide/osmium tetroxide shows that some of these crest-derived cells sprout axons within these sclerotomes. By stage 23, however, no dorsal root ganglia are present within the five occipital sclerotomes, as assessed both by haematoxylin/eosin and zinc iodide/osmium tetroxide staining. Moreover, despite this loss of sensory cells, motor axons grow out in these segments, many of them later fasciculating to form the hypoglossal nerve. The sclerotomes remain visible until stages 27/28, when they dissociate to form the base of the skull and the atlas and axis vertebrae. After grafting occipital neural tube from quail donor embryos in place of trunk neural tube in host chick embryos, quail-derived ganglia do develop in the trunk sclerotomes. This shows that the failure of occipital ganglion development is not the result of some fixed local property of the neural crest or neural tube at occipital levels. We therefore suggest that in the chick embryo the cranial halves of the five occipital sclerotomes lack factors essential for normal sensory ganglion development, and that these factors are correspondingly present in all the more caudal sclerotomes.

The life cycle of the Tantulocarida (Crustacea)

Four new species of parasitic crustaceans belonging to the class Tantulocarida are described, two of which are placed in a new genus,Onceroxenus. Three of them parasitize deep-sea tanaids, the other, a deep sea asellote.Microdajus langi, originally classified as an epicaridean isopod, is recognized as a tantulocaridan. It is reported from Scottish waters for the first time and from new host species. These records include the shallowest depth, 22 m, known for a tantulocaridan.Cumoniscus kruppia parasite of cumaceans, is also recognized as a tantulocaridan. The Tantulocarida now comprises eleven species and five genera, here assigned to the Basipodellidae and two new families, the Deoterthridae and Microdajidae. Several life cycle stages are described and arranged in two developmental sequences. Evidence for a possible third sequence was found. Male development involves a unique type of metamorphosis in which the free-living adult differentiates from a dedifferentiated mass of tissue contained within the expanded trunk of the tantulus larva. Throughout this metamorphosis the male is supplied with nutrients from the host via a tissue connection, the umbilical cord, and the permanently attached larval head. The non-feeding adult male lacks cephalic appendages but possesses two clusters of aesthetascs on its anterior margin. It is free swimming and has six pairs of large thoracopods without endites. The first two thoracic somites are incorporated into the cephalothorax. The abdomen bears a posteriorly directed, median stylet, interpreted as the intromittent organ. It originates on the first abdominal somite. The adult female has a large sac-like trunk attached by the larval head. The larval trunk is sloughed leaving a scar but no complete moult occurs. Eggs develop within the trunk sac and hatch directly at the infective tantulus larval stage. This extreme condensation of early ontogeny is compared with that of other crustaceans and is interpreted as an adaptation to parasitism in situations where a high dispersal ability is not advantageous. In some females the trunk sac forms behind the head but the larval trunk is retained. Small and large females of this type are described, the largest being 737 µm in length. These probably represent females in which sloughing of the larval trunk has failed but it is possible that each may have contained a free-living adult female of comparable size to the adult male. The tantulus larva is described in detail. Scanning electron microscopy reveals that the thoracopodal endites have a complex apical armature, including coupling spines which serve to link the members of a leg pair. Tantulocaridans are permanently attached to their host by the oral disc, presumably by means of an adhesive. In the centre of the disc they make a minute puncture (between 0.5 and 2.0 µm in diameter) through the host integument, probably with the aid of their cephalic stylet. This constitutes their only access to the body fluids of the host. The phylogenetic relationships of the Tantulocarida are discussed. They appear to be related to the barnacles (Thecostraca), both groups possessing a median penis derived from the seventh trunk limb. Their possession of a thorax of six somites and the location of the male gonopores on trunk somite seven suggests an affinity with a larger group containing the Thecostraca and the Copepoda.

A new genus and species of parasitic copepod (Siphonostomatoida: Hatschekiidae) from an Australian Conger eel

Wynnowenia distinctus, a new genus and species of parasitic copepod, is described. It was found on the gills of a conger eel, Congromuraena haberata, in Australian waters. It differs from the other genera of the family Hatschekiidae in the presence of three distinct trunk somites between cephalothorax and trunk, and in swimming leg structure.