Ventrolateral Position Research Articles

Prenatal development of the human nucleus ambiguus during the embryonic and early fetal periods

The ontogenetic development of the nucleus ambiguus was studied in a series of human embryos and fetuses ranging from 3 to 12.5 weeks of menstrual age (4 to 66 mm crown-rump length). They were prepared by Nissl and silver methods. Nucleus ambiguus neuroblasts, whose neurites extend towards and into the IXth and rostral Xth nerve roots, appear in the medial motor column of 4-6-week-old embryos (4.25-11 mm). These cells then migrate laterally (6.5 weeks, 14 mm) to a position near the dorsal motor nucleus of X. At 7 weeks (15 mm), nucleus ambiguus cells begin their migration, which progresses rostrocaudally, into their definitive ventrolateral position. The basic pattern of organization of the nucleus is established in its rostral region at 8 weeks (22.2-24 mm) and extends into its caudal region by 9 weeks (32 mm), when its nearly adult organization is evident. Cells having the characteristics of mature neurons first appear rostrally in the nucleus during the 8.5-9-week period (24.5-32 mm), gradually increase in number, and constitute the entire nucleus at 12.5 weeks (65.5 mm). Definitive neuronal subgroups first appear at 10 weeks (37.5 mm) in the large rostral nuclear region. These features suggest that the human nucleus ambiguus develops along a rostrocaudal temporospatial gradient. Evidence indicates that function of nucleus ambiguus neurons, manifested by fetal reflex swallowing, occurs after the cells migrate into their definitive position, establish the definitive nuclear pattern, and exhibit mature characteristics.

THE SIGNIFICANCE OF THE DEVELOPMENT OF JACOBSON'S ORGAN IN THE NORTH AMERICAN TAILEDFROG ASCAPHUS TRUEI(ANURA: LEIOPELMATIDAE)

ABSTRACT The North American tailed frog (Ascaphus truei) is presently regarded as one of the primitive members of the Anura owing to the possession of the following primitive characteristics: free bony ribs, the retention of two ‘tail-wagging’ muscles and Jacobson's organ, which in position corresponds to that of more advanced urodeles. Unlike most other anurans, the tailed frog lacks a tympanum and is voiceless. According to Jurgens (1971), the tendency exists for Jacobson's organ in primitive urodeles to migrate from a position ventral to the main nasal sac to a ventrolateral position in phylogenetically advanced urodeles. Furthermore, in primitive anurans Jacobson's organ is situated ventrolaterally to the main nasal sac and the tendency exists for it to shift anteromedially to lie between the septum nasi and main nasal sac in advanced anurans. Because of this character shared by Ascaphus and advanced urodeles, the embryological ventrolateral shifting of the organ in Ascaphus truei could be regarded as a reflection of the hypothetical evolutionary ancestral transition between aquatic and terrestrial vertebrates. Jurgens' opinions on the phylogenetic migration of Jacobson's organ are corroborated by the embryological development of the organ in Ascaphus truei.

Dendritic ramifications of trigeminal motor neurons innervating jaw-closing muscles of rats

A cholera toxin subunit conjugated horseradish peroxidase (CTHRP) was found to be very useful in labelling the dendrites of motoneurons. CTHRP was injected individually to jaw-closing muscles (temporalis, masseter, and medial pterygoid) of rats, and their motoneurons including the dendrites were labelled and studied. The results show that the motoneuron cell bodies innervating temporalis, masseter, and medial pterygoid muscles are located in the trigeminal motor nucleus in dorsal, ventromedial and ventrolateral position. The dendrites of these motoneurons extend radially into mesencephalic nucleus, supratrigeminal nucleus, pontine reticular formation, trigeminal sensory nucleus and even into the bundles of the ascending root of the facial nerve. These dendrites may serve as an extended surface for various synaptic contacts to the jaw closing motoneurons. The possibility that they may also have presynaptic influence on the input to the trigeminal motoneurons is also discussed.

The motor nuclei and sensory neurons of the IIIrd, IVth, and VIth cranial nerves in the monitor lizard, Varanus exanthematicus.

The motor nuclei of the oculomotor, trochlear, and abducens nerves of the reptile Varanus exanthematicus and the neurons that subserve the sensory innervation of the extraocular muscles were identified and localized by retrograde and anterograde transport of horseradish peroxidase (HRP). The highly differentiated oculomotor nuclear complex, located dorsomedially in the tegmentum of the midbrain, consists of the accessory oculomotor nucleus and the dorsomedial, dorsolateral, intermediate, and ventral subnuclei. The accessory oculomotor nucleus projects ipsilaterally to the ciliary ganglion. The dorsomedial, dorsolateral, and intermediate subnuclei distribute their axons to the ipsilateral orbit, whereas the ventral subnucleus, which innervates the superior rectus muscle, has a bilateral, though predominantly contralateral projection. The trochlear nucleus, which rostrally overlaps the oculomotor nuclear complex, is for the greater part a comma-shaped cell group situated lateral, dorsal, and medial to the medial longitudinal fasciculus. Following HRP application to the trochlear nerve, almost all retrogradely labeled cells were found in the contralateral nucleus. The nuclear complex of the abducens nerve consists of the principal and accessory abducens nuclei, both of which project ipsilaterally. The principal abducens nucleus is located just beneath the fourth ventricle laterally adjacent to the medial longitudinal fasciculus and innervates the posterior rectus muscle. The accessory abducens nucleus has a ventrolateral position in the brainstem in close approximation to the ophthalmic fibers of the descending trigeminal tract. It innervates the retractor bulbi and bursalis muscles. The fibers arising in the accessory abducens muscles form a loop in or just beneath the principal abducens nucleus before they join the abducens nerve root. The afferent fibers conveying sensory information from the extraocular muscles course in the oculomotor nerve and have their perikarya in the ipsilateral trigeminal ganglion, almost exclusively in its ophthalmic portion.

Organization within the cranial IX-X complex in ranid frogs: a horseradish peroxidase transport study.

Cranial nerves IX and X in frogs have been described as originating from a nuclear group referred to as the IX-X complex. We studied the central nervous system components of this complex in Rana pipiens and R. catesbiana by labeling peripheral branches of cranial nerves IX and X and identifying the central nervous system contributions of these branches. Various peripheral nerves (IX and the cardiac, gastric, pulmonary, and laryngeal branches of X) were identified and soaked in horseradish peroxidase (HRP). One to 2 weeks later, the frogs were killed and processed for HRP by the tetramethylbenzidine method. Glossopharyngeal efferents originated from a small ventrolateral cell group found at the level of IX root exit. Vagal efferents formed a single column of cells in a ventrolateral position from the level of the brainstem exist of the vagus nerve (approximately 2,000 micrometers above the obex) to 200 micrometers below the obex (values given are for an 80-g frog). This cell group was separate from and just caudal to efferent cells of the glossopharyngeal nerve. Within the vagal portion of the column, cells projecting through the gastric branch were found throughout the rostral-caudal extent of the nucleus. "Cardiac" cells tended to be more rostral than "pulmonary" cells, and both groups of cells were located in the middle of the nucleus. "Laryngeal" cells were located more caudally in the nucleus. This peripheral representation within the vagal nucleus corresponds more closely to the organization found in the mammalian nucleus ambiguus, rather than to the apparent lack of organization found in the mammalian dorsal motor nucleus. Afferents of IX and X entered slightly rostral to the ventral roots of their respective nerves and descended in two tracts. The majority entered the tractus solitarius and descended in a medial position to cervical spinal cord. A portion of the afferents from the vagus nerve crossed the midline in the lower myelencephalon just dorsal to the central canal and ascended a short distance on the contralateral side. Within the solitary tract, vagal afferents were located in a ventrolateral position as they descended to below the obex. Glossopharyngeal afferents filled the remainder of the tract. A smaller portion of afferents from both IX and X did not enter the solitary tract but descended in the spinal tract of V and the dorsolateral funiculus of the spinal cord (Lissauer's tract) to thoracic levels. Afferents of IX also formed a rostral bundle which extended in the solitary tract to the caudal metencephalon.

Death of motorneurons during the postnatal loss of polyneuronal innervation of rat muscles.

A study was made of the decline in the number of motor neurons and axons of the brachial spinal cord of the rat during postnatal development. The injection of horseradish peroxidase (HRP) into the biceps muscle showed that it was innervated by motor neurons located in the dorsolateral position of the lateral motor column in segments C5 and C6; HRP injections into the triceps muscle showed that it was innervated by motor neurons located in the ventrolateral position of the lateral motor column in segments C7 and C8. There was no change in the position of these motor neuron pools between birth and maturity. However, there was a decline in the number of neurons in each pool during the postnatal period; over 35% of the neurons present at birth had disappeared by maturity. This loss of neurons was uniform throughout the rostrocaudal extent of each pool. It was accompanied by a similar percentage loss in the number of axons in a ventral root at the branchial level (C8). Electrophysiological measurements showed that the disappearance of motor neurons was accompanied by a loss in the polyneuronal innervation of synaptic sites in the biceps muscle. The possibility that a decrease in the number of neurons contributes to the loss of polyneuronal innervation is discussed.

Brain stem influences on the parasympathetic supply to the urinary bladder of the cat

(1) The brain stem of anaesthetized cats has been mapped between Horsley-Clarke planes APO and P8.5 with electrical stimuli of low intensity in order to determine the areas which can produce excitatory or inhibitory influences on the spontaneously contracting, sympathetically denervated, urinary bladder. (2) Two inhibitory areas were found. The first extended from P3.0 to P8.5, and at all levels was coincident with the midline raphe nuclei. The second area occured largely 2–3 mm lateral to the midline, in the area of the nucleus reticularis pontis caudalis rostrally, and the nuclei reticularis gigantocellularis and parvocellularis caudally. Both of these areas were found to inhibit bladder contractions with threshold stimulus parameters of 20–60 μA, 400 μS, 20Hz. (3) One excitatory area was found, largely 3–4 mm lateral to the midline. This area appeared large and diffuse in the lateral reticular formation. It is possible that it originated in the pontine micturition centre in the rostral pontine tegmentum. Caudally, it shifted to occupy a ventrolateral position. This excitatory area was in close approximation to, and was probably interspersed with, the lateral inhibitory area. (4) In decerebrate preparations the areas that produced excitation or inhibition had the same distribution as those found in anaesthetized animals. (5) Single shock stimulation (100 μA, 400 μs, 0.5 Hz repetition frequency) in the excitatory area could produce firing in pelvic nerve efferents to the bladder at latencies of 60–110 ms. The amplitude of such responses was dependent on the level of intravesical pressure. (6) Stimulation of the inhibitory areas produced no evoked responses in the pelvic nerve efferents, but could inhibit reflexly evoked responses in this nerve. The siimilarity in the time courses of the inhibitory effects from the two areas raises the possibility that one acts via the other.

Cytoarchitectonic organization of lumbar preganglionic sympathetic neurons in the cat.

The location and dendritic projection of lumbar preganglionic sympathetic neurons (PSN) has been investigated in the cat. Horseradish peroxidase was either injected into lumbar paravertebral ganglia or directly applied to a lumbar ramus communicans albus. PSN were regularly found in the lateral funiculus and in the intermediate area of the ipsilateral spinal cord. They were concentrated in different nuclei: nuclei intermediolateralis pars principalis (IMLp) and pars funicularis (IMLf), nucleus intercalatus (IC) and medially near the central channel. Inconsistently, some PSN were observed in the ventral horn. Dendrites extending over a few soma were found in nearly all PSN, thus allowing description of the main dendritic projections of the preganglionic nuclei. Sometimes axons of IMLp and IC neurons could be traced into the ventral roots. In the IMLp the PSN were arranged in round clusters (up to 30 cells), which regularly followed each other in rostrocaudal direction. These clusters increased their mediolateral diameter from L1 to L4. Many cells distributed their dendrites longitudinally within the IMLp, but dendritic projections into the dorsal horn, the intermediate area and into the lateral funiculus were also frequently observed. The IMLf consisted of a few PSN only. These neurons were located in the tissue spaces between axon bundles and mediolaterally arranged, pointing towards the IMLp. The PSN of the IC were mediolaterally oriented and arranged in transverse bands extending from the lateral horn to the dorsal aspects of the central channel. All cells had mediolaterally oriented dendrites which sometimes crossed the spinal cord midline in case of the medially located neurons. The dendrites of the laterally located cells projected very often into the dorsal horn. In the medial region the PSN covered a narrow area extending from the dorsal aspects of the central channel in a ventrolateral direction up to the level of the ventral commissure. The neurons were localized in clusters and mediolaterally or longitudinally oriented. Dense clusters consisting of round cells were prevalent in the ventrolateral position. The dendrites had a rostrocaudal and mediolateral orientation and, in case of the dorsomedially located cells, sometimes crossed the spinal cord midline. The main dendritic projection areas of the different preganglionic nuclei have been compared with the location of the terminal fields of descending supraspinal and propriospinal systems and of primary afferents. It has been considered that this structural arrangement would allow for intricate interaction of the various systems with subpopulations of PSN. Thus each preganglionic nucleus might integrate specific neuronal information.

The organization of the extraocular motor nuclei in the Atlantic stingray, Dasyatis sabina.

Retrograde transport of HRP was used to determine the location and organization of the motor nuclei innervating the extrinsic eye muscles of the stingray, an elasmobranch fish. oculomotor neurons are located both medial to and immediately ventrolateral to the MLF in the rostral midbrain. A ventral oculomotor nucleus was found among the IIIrd nerve rootlets close to the base of the midbrain. The dendrites of cells in the dorsal nucleus appear to be preferentially oriented in the transverse plane penetrating the MLF. Motoneuron pools innervating individual muscles are incompletely segregated in the dorsal group. However, the ventral nucleus innervates only the inferior oblique muscle. Dorsally, motoneurons innervating a single muscle are found on both sides of the MLF. In the caudal midbrain, the majority of trochlear motoneurons are located immediately ventrolateral to the MLF. Abducens motoneurons are scattered in the medulla from a ventrolateral position resembling the location of the nucleus in teleost fish to a dorsomedial position close to the MLF as in most other vertebrates. In contrast to other vertebrates, the medial rectus muscle is innervated by the contralateral oculomotor nucleus. Motoneurons innervating the other muscles have the same laterality as found in other vertebrates.

Origin, course and terminations of the rubrospinal tract in the pigeon (Columba livia).

The red nucleus and its spinal projections in the pigeon (Columba livia) have been studied using both normal and experimental material. The cytoarchitecture of the nucleus is described on the basis of Nissl-stained sections and reveals an organization generally similar to that of mammals. The large neurons (40-50 mum) tend to be located dorsomedially and ventrolaterally at more caudal nuclear levels, while the small- and medium-sized neurons (15-35 mum) predominate at rostral levels. However, neurons of all sizes are present throughout the nucleus. Following lesions of the nucleus, the course of degenerating axons stained with the Fink-Heimer method has been traced throughout the brainstem and spinal cord. The rubrospinal tract crosses the midline, courses past the ventrocaudal aspect of the contralateral nucleus ruber, and then descends rostro-ventral and lateral to the nucleus tegmenti pontinus. In its caudal continuation the tract lies ventral to the brachium conjunctivum and the entering radix of the trigeminal nerve. It then assumes a ventrolateral position in the caudal brainstem before shifting to a dorsolateral position in the lateral funiculus of the spinal cord. Within the spinal grey the rubrospinal tract terminates in laminae V, VI and to a lesser extent VII. The possibility of a topographical organization of the nucleus was investigated with injections of horseradish peroxidase into brachial, thoracic and lumbar spinal cord. Regardless of the level of injection, labelled neurons of all sizes were present throughout the contralateral nucleus ruber, indicating the absence of an obvious topography.

Edinger-Westphal nucleus: Projections to the brain stem and spinal cord in the cat

The efferent connections of the Edinger-Westphal (EW) nucleus of the cat have been examined using the autoradiographic anterograde axonal transport technique. Following injections of [ 3H]amino acids into the EW nucleus, fibers could be traced from this region to a number of sites in the caudal brain stem and spinal cord heretofore not known to receive an afferent input from this nucleus. Two descending pathways have been identified. One pathway travels in the medialmost aspect to the medial longitudinal bundle and terminates in the dorsal accessory olive. The other pathway leaves the nucleus laterally, coursing through the medial tegmentum, and then shifts to a ventrolateral position in the rostral rhombencephalon. Some fibers of this lateral pathway curve dorsally and terminate in the medial parabrachial nucleus. The remainder of this fiber system lies ventral to the spinal trigeminal complex, with some axons terminating in the subtrigeminal nucleus, while other fibers continue ventral to the caudal part of the spinal trigeminal nucleus and appear to terminate in the marginal layer and ventromedial part of this nucleus. Another component of this system terminates between the gracile and medial cuneate nuclei. The main pathways from the EW nucleus to the spinal cord include (1) some fibers which course through the dorsal column nuclei into the ventromedial part of the dorsal columns, and (2) other fibers which continue caudally immediately along the ventrolateral aspect of the spinal trigeminal nucleus and then proceed to the spinal cord in the region between the dorsal horn and the lateral cervical nucleus. These EW fibers appear to terminate mainly in Rexed's lamina I (marginal layer); other fibers, from both tracts appear to terminate in lamina V. There was no evidence for any ascending projections from the EW nucleus. Confirmatory data were obtained from a series of horseradish peroxidase (HRP) experiments, in which EW neurons were retrogradely labeled following injections into the dorsal column nuclei, spinal trigeminal nucleus, inferior olivary nucleus, or spinal cord. These results clearly indicate that the traditional view of the EW nucleus as merely a parasympathetic preganglionic nucleus should be seriously questioned.

The motor column and sensory projections of the branchial cranial nerves in the frog.

AbstractMotor nuclei and primary afferent projections of branchial cranial nerves were shown with the CoCl2 staining technique. Motor nuclei constitute a discontinuous column at the ventrolateral margin of the gray matter. The size of motoneurons decreases in the orocaudal direction within the column. The dendrites assemble in two main directions forming a dorsomedial and a ventrolateral dendritic array. The axons originate in the ventrolateral dendrites and run straight to the motor roots. Preganglionic vegetative neurons were found in a separate group at the level of the abducens nucleus, and intermingled with motoneurons at the level of the glossopharyngeus and vagus nuclei. Their axons join the visceral sensory roots of the respective nerve.The tractus spinalia trigemini has a short and weak ascending limb, and a strong descending limb which decussates in the first and second segments of the spinal cord. A few uncrossed large fibers continue as far as the midthoracic level, In addition to trigeminal fibers, a small contingent of facial fibers and a significant number of vagus fibers contribute to the tractus spinalis. Small caliber fibers of the tractus and fine collaterals of large caliber fibers terminate in a continuous column which lies, in its largest part, in the neuropil medial to the tractus. The pattern of fiber terminations in this area is identical with that in the substantia gelatinosa. This area is called the nucleus spinalis trigemini. Coarse collaterals terminate in a discontinuous column of large and medium‐sized neurons, extending caudally to the obex region. This area is the largest at the level of the trigeminus nucleus and is called the nucleus principalis trigemini.The cells of origin of the mesencephalic tractus of the trigeminus lie in the second and fourth layers of the optic tectum. The descending fibers run ventral to the tractus spinalis in a separate bundle. They emit collaterals to the large fiber termination areas and to branchiomotor nuclei. Direct contacts are established with trigeminus motoneurons. In the obex region a significant number of collaterals invade the dorsomedial part of the gray matter.The fasciculus solitarius begins at the level of the trigeminal nucleus and terminates in the commissura infima at the medullospinal border. The nucleus solitarius begins at the oral pole of the glossopharyngeus nucleus. The fasciculus receives fibers from the facialis (intermedius), glossopharyngeus and vagus nerves. The fibers spread over the total extent of the fasciculus: the facialis fibers exhibit a preference for a dorsomedial position and the glossopharyngeusvagus fibers for a ventrolateral position. There are several interchanging fibers between the somatic sensory and visceral sensory projecting areas.The results indicate a close similarity between amphibian and mammalian brain stem motor nuclei and sensory projections, though some parts are present in a primordial form in the frog.

The structure and properties of an abdominal chordotonal organ in Carausius morosus and Blaberus discoidalis

Each side of the abdominal segments of the stick insect Carausius morosus contains a chordotonal organ lying longitudinally in a ventro-lateral position. These ventro-lateral chordotonal organs each possess two nerve cell bodies and two scolopales. There is a single attachment strand to the cuticle. Electrical recordings from the receptors show that they respond in a highly phasic manner to both stretching and subsequent relaxation of the attachment strand. They are sensitive to substrate vibration but are activated by ventilatory movements. The effects of ramp and square wave stimulation are examined. The rôle of the ventro-lateral chordotonal organs as ventilatory receptors is discussed and abdominal chordotonal organs of insects in general are reviewed. The ‘ventral phasic receptors’ of the cockroach are re-examined and shown to be chordotonal organs. They are re-named ‘mid-ventral’ chordotonal organs.

SEX ATTRACTION AND COURTSHIP BEHAVIOR IN LAMBDINA FISCELLARIA LUGUBROSA (LEPIDOPTERA: GEOMETRIDAE)

AbstractCalling by female Lambdina fiscellaria lugubrosa (Hulst) occurred when terminal abdominal segments 8–10 were protracted, exposing an enlarged region in the intersegmental membrane between segments 8 and 9. Histological examination of this region disclosed paired glands situated ventrolaterally in the eighth segment. The glands were presumed to be pheromone glands, and differ from those described for other Lepidoptera, in their paired structure and ventrolateral position. More males were attracted to traps containing virgin females than to empty control traps, but only during the first half of the moth flight. Board and yellow carton sticky traps were superior to 3M and white carton traps. Behavioral observations suggested that the sex pheromone may serve as an excitant as well as, or instead of, an attractant, stimulating the male to searching activity.

A radioautographic analysis of the migration and fate of cells derived from the occipital somites in the chick embryo with specific reference to the development of the hypoglossal musculature

ABSTRACT The migration pattern and fate of cells of the occipital somites and overlying ectoderm have been described for the chick embryo with particular reference to the development of the hypoglossal musculature. Tritium-labelled thymidine (0·5–10 μCi per egg) was used as a cell-specific marker. Occipital somites (2–5) with overlying ectoderm were transplanted orthotopically from labelled donor embryos to unlabelled host embryos (Hamburger & Hamilton, stage 9–10). The embryos were incubated, for varying lengths of time (24 h–5 days), sacrificed, sectioned and the migration pattern and fate of the labelled cells determined radioautographically. It appears that the hypoglossal as well as other hypopharyngeal musculature originates from the occipital somites. The mesodermal migration pattern extended from the occipital somite region in a ventro-posterior direction to the dorsal surface of the pericardial cavity posterior to the expanded portion of the pharynx. At this position a so-called hypoglossal cord formed on each side which ran anteriorly to the level of the second pharyngeal pouch where it turned medially and together with the cord from the other side entered the pharyngeal area of the embryo. This material apparently forms the intrinsic musculature of the tongue. The mesodermal movements are attributed to differential growth movements of the areas concerned as well as to active cell mutiplication and migration. Selective embryonic neuronal staining was undertaken to study the relationship between the migrating hypoglossal cord and nerve. The cord preceded the nerve in its migration. There is an occipital somitic contribution to the primitive meninx, to the endothelial walls of developing blood vessels, possibly to microglial cells and to the cartilage surrounding the notocord. The occipital ectoderm expands dorso-anteriorly and ventro-laterally. Tn the ventro-lateral position as contact is made with the pharyngeal endoderm a placode is formed which contributes cells to the nodose ganglion of the tenth cranial nerve. There is no other contribution of the ectoderm to the underlying tissues.

Neurosecretion in the Indian scorpion Buthus tamulus

Neurosecretory cells in the brain and second abdominal ganglion of the pregnant female Indian scorpion ( Buthus Tamulus) have been studied. Neurosecretory cells occur not only in the median pars intercerebralis, but also in the lateral and ventral part of the protocerebrum and are grouped in five loci. They are of different sizes and show different cyclical activity. Medium and large-sized neurosecretory cells packed with PF-positive granules seem to be correlated with the presence of embryos in the uteri. Neurosecretory cells of the protocerebrum have been grouped in five loci. The possibility of recruitment of neurons from the globulus region is presented. Mitotic activity is observed in the ganglion mother cells in the globulus region of the protocerebrum of the adult scorpion. Two median nerves homologous to the nervi corporis cardiaci interni of insects occur in the protocerebrum; their axons contain neurosecretory material. Neurosecretory cells are present in the second abdominal ganglion in the anterolateral, lateral, and ventrolateral positions. They discharge neurosecretory material into the supraneural artery.

Studies on the Biology of Elachertus ceramidiae (Hymenoptera: Eulophidae), a Parasite of the Banana Pest Ceramidia butleri (Lepidoptera: Syntomidae)1

Elachertus ceramidiae (Boucek), first described as a parasite of Ceramidia viridis Druce in Ecuador, is an important parasite of C. butleri Moschler in Central America. In Costa Rica the complete life cycle took about 13 days in the insectary. The 8–28 eggs deposited per female were attached externally on or near the host's mid-dorsal line and hatched in 2 days, larvae pupated on the seventh posthatch day, and adults emerged about 4 days later. Host caterpillars usually died on the parasites' fifth posthatch day, having fed but little on the 2 preceding days. The parasite larvae were not observed to molt. As they matured, they moved to ventrolateral positions on the host for pupation. Commonly more eggs were deposited on a host than would survive to adulthood, and often several females oviposited on the same host. Maximum egg counts were 118 on a field-collected host, 123 on one parasitized in the laboratory. The sex ratio of E. ceramidiae reared in the insectary was 1 ♂ to 9 ♀.

APLASIA OF THE OPTIC NERVE

Aplasia of the optic nerve is a congenital anomaly rarely seen except in grossly malformed eyes. Cords 1 (1923) said that it is one of the greatest rarities. A brief review of the embryology of the optic nerve will facilitate understanding the condition. 2 By the 2.6 mm. stage of the embryo (fig. 1) the optic pits are becoming visible on each side of the anterior end of the neural canal. These pits rapidly grow out, forming by the 4 mm. stage the hollow, bulbous optic vesicles, the cavities of which are connected with the lumen of the neural canal by the short optic stalks. At the 4.5 mm. stage (fig. 2) the fetal fissure begins to form by invagination of the optic vesicles in a ventrolateral position. The outer wall of each vesicle, which is to become the retina, has by this time increased to several cell layers in