Somatic Fibers Research Articles

An immunohistochemical study of putative neuromodulators and transmitters in the centrifugal visual system of the quail ( Coturnix japonica)

The aim of the present study was to analyze the neurochemical properties of the centrifugal visual system (CVS) of the quail using an immunohistochemical approach by testing 16 neuropeptides (angiotensin: ANG, bradykinin: BK, cholecystokinin, dynorphin, L and M-enkephalin, β-endorphin: β-END, galanin, α-neoendorphin, neurokinin A, neuropeptide Y (NPY), ocytocin, somatostatin, substance P, vasopressin, vasoactive intestinal polypeptide) and three neurotransmitters or their synthetic enzymes (choline acetyltransferase: ChAT, tyrosine hydroxylase: TH, serotonin: 5-HT and nitric oxide synthase: NOS, including the histochemical nicotinamide adenine dinucleotide phosphate diaphorase technique). For each substance, the somatic and afferent fiber and terminal labeling was analyzed within the nucleus isthmo-opticus (NIO) and the ectopic area (EA) and compared with that of retinopetal cell bodies labeled retrogradely with RITC following its intraocular injection (double-labeling procedure). The results showed that none of the centrifugal neurons were reactive to any of the substances tested. In contrast, all with the exception of ANG, BK and β-END, labeled fibers and terminals within the EA and only four (ChAT, 5-HT, NPY and NOS) within the NIO. Possible sources of these immunoreactive fibers terminating in the NIO and EA were investigated by mapping the somatic immunolabeling of the different substances within brainstem regions previously shown by Miceli and other authors to project upon the centrifugal neurons. The data suggests that, besides the rapid retino-tecto-NIO-retinal loop, which facilitates the transfer of meaningful or more relevant information within particular portions of the visual field, the multiple afferent input which stems from various brainstem regions utilizes a wide range of neuroactive substances. Some of these afferent projections upon the centrifugal neurons appear to belong to nonspecific systems which might play a role in modulating the excitability of centrifugal neurons as a function of arousal.

Preliminary study of large and small peripheral nerve fibers in Charcot-Marie-Tooth disease, type I.

Histologic studies of Charcot-Marie-Tooth disease, type I, show a contrast between the lesions of myelinated fibers and the normality of unmyelinated fibers. Conventional electrophysiologic tests only demonstrate the alteration of myelinated fibers but do not study unmyelinated fiber function. We present routine clinical tests that are easily available and effective for the evaluation of small unmyelinated fibers: thermal threshold testing for warmth to evaluate small C unmyelinated somatic fibers and sympathetic skin responses to evaluate small C unmyelinated sympathetic fibers. Five unrelated patients with a diagnosis of Charcot-Marie-Tooth disease, type I, confirmed by biopsy were investigated. All of these patients showed marked reduction or absence of motor and sensory conduction velocities and severe denervation at needle examination. By contrast, thermal threshold testing for warmth and sympathetic skin responses were normal, confirming the normality of small C unmyelinated somatic and sympathetic fibers. We conclude that these noninvasive tests are helpful in the diagnosis of Charcot-Marie-Tooth disease, type I.

Ascending spinal influences on rubrospinal cells in the cat.

Somaesthetic input to rubrospinal cells, bypassing the cerebellum and cerebral cortex, has been demonstrated in the cat. The detailed organization of this somatic afferent system was studied using electrophysiological methods on multiple-lesion, chloralose-anaesthetized preparations. Stimulation of the dorsal column (DC) at upper cervical cord segments induced significant responses in magnocellular red nucleus (RNm) cells in cats without a cerebellum and with ablation of the frontal cortex. As classic descriptions state that primary afferent fibres have ascending and descending branches in the DC, with many collaterals arborizing in the grey matter at the segmental level of the cord, this procedure is equivalent to stimulating the somatic fibres coming from a large portion of the body, leading to the simultaneous activation of most ascending spinal pathways. To show that the pathway responsible for the rubral responses ascends in the ventral spinal cord, and that the synaptic relays are located at the segmental level, the stimulation was applied to the DC, caudally to the sectioned dorsal spinal half. Various tests confirmed that the activation was conducted to rubral cells through antidromically activated primary afferents. Their multiple collaterals relay the messages to cells located caudal to the spinal lesion, with fibres ascending in the ventral cord. Any relay of the somatic rubral responses in the DC's nuclei was excluded. When the DC was sectioned and its rostral end was dissected free and lifted onto two hook electrodes for stimulation, no response was obtained in the rubral cells. This dissection indeed sectioned all DC fibre collaterals entering the grey matter, thus excluding the possibility of segmental relay. Single shocks applied to the ventral quadrant of the cord or in the medial lemniscus (LM) in the medulla oblongata induced monosynaptic excitatory post-synaptic potentials (EPSPs) in most rubrospinal cells. The spinal EPSPs could be collided by stimulation in the LM, thus demonstrating the existence of direct connections from the cord to the RNm. This somaesthetic pathway to the RNm could be involved in on-line correction of movements and in learning new motor strategies.

Role of the nervi nervorum in neuropathic pain: Innocent until proven guilty

D octors Sove and Light present an important and interesting hypothesis on the function of unmyelinated sensory fibers with free endings in the perineurium. They suggest that these nociceptive fibers of the nervi nervorum serve an integral and unique role in the pathogenesis of neuropathic pain by signaling pain from direct stimulation and by initiating and reinforcing the electrophysiologic events that characterize the neuropathic pain state. Their hypothesis is based on several of their own observations and a substantial literature on the anatomy, function, and chemical sensitivity of nerve fibers. As the authors point out, the hypothesis has been expressed in slightly different forms in the past, but has not been formally tested. It extends a more fundamental hypothesis suggesting that the primary role of the nervi nerverum is to guard the fascicular axons from physical injury by nociceptive signaling and to help regulate the endoneurial environment through the release of neuropeptides that alter the blood-nerve barrier. That the nervi nervorum may contribute directly to the pathogenesis of neuropathic pain is an appealing hypothesis, but one that is difficult to test given the technical problems of isolating perineurial fibers and stimulating them without affecting the very large number of adjacent somatic fibers in the fascicular endoneurial space. To think that compression of the perineurium or chemical alterations in its environment could cause aberrant firing in the terminal receptors of the nervi ner-

Neurological testing in erectile dysfunction.

Neurophysiologically, penile erection is elicited by two different mechanisms and mediated by somatic and autonomic pathways. Psychogenic erections, initiated in supraspinal centers in response to auditory, visual, olfactory, and imaginative stimuli, are mediated by sympathetic pathways. Reflexogenic erections, elicited by tactile stimulation at the genital level, are mediated by a spinal reflex arc consisting of afferent somatic and efferent parasympathetic nerve fibers. Therefore, the neurological factor in erectile dysfunction (ED) may include central and peripheral neural structures (Andersson and Wagner, 1995; Giuliano et al, 1995). The ideal neurourophysiological assessment will objectively and quantitatively evaluate the functional status of all parts of this neurological network. In the last two decades, a series of tests has been developed, each of which reflects a specific part of the network. This review gives a brief overview of neuroanatomy and physiology of the erectile response and focuses on current applicability of neurophysiological tests for evaluating the integrity of the nervous system involved in erectile function.

Selective Detrusor Activation By Electrical Sacral Nerve Root Stimulation in Spinal Cord Injury

Electrical sacral nerve root stimulation can be used in spinal cord injury patients to induce urinary bladder contraction. However, existing stimulation methods activate simultaneously both the detrusor muscle and the urethral sphincter. Urine evacuation is therefore only possible using poststimulus voiding. Micturition would improve if the detrusor muscle could selectively be activated. The purpose of this study was to demonstrate selective detrusor activation in patients by ventral sacral root stimulation. The stimulation method involves selective activation of the small diameter myelinated nerve fibers and consists of a combination of cathodal excitation and selective anodal blocking using a tripolar electrode. To investigate anodal blocking, the intraurethral pressure response to stimulation was measured in acute experiments performed on 12 patients. The influence of both pulse amplitude and pulse duration on the pressure response was analyzed. In 8 out of 12 patients anodal blocking of somatic motor fibers was possible. This study also indicates the feasibility of selective detrusor activation by sacral root stimulation.

Misexpression ofnautilusInduces Myogenesis in Cardioblasts and Alters the Pattern of Somatic Muscle Fibers

nautilus(nau), one member of the myogenic regulatory family of bHLH-encoding genes, is expressed in a subset of muscle precursors and differentiated fibers in theDrosophilaembryo. To elucidate the role ofnautilusin myogenesis, we have misexpressed it using the GAL4-targeted system. We find that ectopic expression results in lethality throughoutDrosophiladevelopment. We analyzed the effects of embryonic expression in mesodermal tissues that include the cardioblasts of the dorsal vessel as well most, if not all, of the presumptive somatic muscle precursors. Immunohistochemical staining for muscle MHC revealed abnormalities that include an absence of cardial cells, coincident with the appearance of novel muscle fibers adjacent to the dorsal vessel. Moreover, many cardioblasts express increased levels of muscle-specific genes such as myosin, actin 57B, and Mlp60A, a protein that is restricted to the somatic, visceral, and pharyngeal muscles. These data suggest that the missing cardial cells have been transformed into cells with properties similar to those of the somatic muscles. In addition, ubiquitous expression ofnautilusin somatic muscle cells of these embryos resulted in muscle pattern defects. Specifically, muscles that do not normally expressnautiluswere frequently absent, and novel fibers were observed in positions reminiscent ofnau-expressing muscles. These data imply thatnautiluscan alter the developmental program of muscle precursors. In summary, we suggest thatnautilusinduces myogenic differentiationin vivowhen ectopically expressed in developing cardioblasts and may affect the myogenic differentiation program of specific muscle fibers.

Synaptic organization of amphibian sympathetic ganglia.

The synaptic organization of the amphibian sympathetic ganglia was studied, especially in the last two abdominal paravertebral ganglia of the frog. These ganglia appear to form a monosynaptic relay, not containing interneurons. They consist of two systems working in parallel: the principal neurons, by far the most numerous, and a small number of chromaffin (i.e., SIF) cells, usually arranged in clusters. Each principal neuron is innervated by a preganglionic branch forming a set of cholinergic synapses which exhibit classical ultrastructure. The only peculiarity is the presence of a subsynaptic apparatus in a variable percentage of synaptic complexes. Electrophysiological studies have demonstrated that synaptic transmission is due to ACh release and involves several postsynaptic potentials. Moreover, the principal neurons are of two types, B and C, whose preganglionic axons and their own axons have different conduction velocities. C neurons tend to be small in diameter, and B neurons are larger, but the size distribution of the two populations overlaps. More recently, it was demonstrated that these two neuronal systems have different immunocytochemical features. The C preganglionic fibers contain an LHRH-like peptide, which is responsible for late synaptic events. The B preganglionic fibers contain CGRP, whose role has not yet been established. The principal neurons all contain adrenaline, but neuropeptide Y is also present in C neurons and could be a second transmitter at peripheral junctions. SP-containing fibers also pass through the ganglia, but give rise to intraganglionic synapses only rarely, except in the celiac plexus. Galanin can coexist with neuropeptide Y in certain C neurons. Numerous principal neurons are immunoreactive for VIP. Chromaffin cells contain noradrenaline and metenkephalin, and some contain SP or LHRH; they are endocrine cells controlled by preganglionic fibers and can have a modulatory effect on principal neurons endowed with appropriate receptors. The accessibility of frog abdominal ganglia and the anatomical separation of B and C preganglionic fiber pathways provide interesting systems in which to carry out experimentation on the stability and specificity of synaptic contacts. After postganglionic axotomy, the majority of synapses disappear by disruption of synaptic contacts. There is a certain discrepancy between the recovery of synaptic transmission and the reappearance of morphologically identifiable synapses, suggesting that a certain amount of transmission is possible at contacts devoid of synaptic complexes. The selective deafferentation of B or C neurons showed that the subsynaptic apparati are mainly found at B neuron synapses. The course of reinnervation following selective deafferentation reveals the existence of different specificities at B and C synapses: C neurons are easily reinnervated by B preganglionic fibers, whereas C fibers appear fairly ineffective at reinnervating B neurons, even after a long interval. Attempts were made to reinnervate ganglionic neurons with somatic motor nerve fibers. Reinnervation was achieved only rarely, and it is concluded that the ganglionic synapses in the frog have a higher specificity and lower plasticity than in mammals.

Both motor and sensory abnormalities contribute to changes in foot posture in an experimental rat neuropathic model

This study was undertaken to find out whether pain contributes to abnormal posture of the foot in rats displaying neuropathic pain behaviors, and if so, whether there is a specific abnormality which reflects the level of pain. We used a rat model of neuropathic pain which is produced by tightly ligating the L5 and L6 spinal nerves just distal to the dorsal root ganglion (DRG). The operation produced neuropathic pain behaviors along with abnormal foot posture as has been shown previously. On the other hand, injury to the same somatic sensory and motor fibers at a more proximal site by sectioning dorsal and ventral roots of the same segments did not produce neuropathic pain behaviors. Rats with dorsal and ventral rhizotomies showed an abnormality of foot posture despite the absence of neuropathic pain behaviors. However, the abnormality in these rats was less severe and lasted for a shorter period of time as compared to rats with spinal nerve ligation. This suggests that peripheral nerve injury in our rat neuropathic pain model produces a foot postural abnormality partially due to motor deficit. However, there is an additional component of the abnormality that is related to sensory function, possibly pain. Close examination of the data indicate that there is no single obvious abnormal foot posture which exclusively represents motor deficit or sensory abnormality. Rather, neuropathic rats are likely to display abnormal foot posture as a result of a complex mixture of motor and sensory abnormalities.

Penile Thermal Sensation

To our knowledge no direct measurement of autonomic failure in the penis is currently available. Indirect techniques in clinical use, such as bulbocavernosus reflex, genital somatosensory evoked potentials and biothesiometry, all rely on large nerve fiber function. Since micturition and potency depend on small fibers of the peripheral system, tests of these fibers might be more relevant in the clinical diagnostic evaluation. We provide upper normal limits and demonstrate repeatability of small fiber mediated sensations of warm and cold sensory thresholds on penile skin. Penile warm and cold sensory thresholds were measured in 35 healthy volunteers (at 2 sessions in 27) via 2 methods. Upper normal values, expressed as 95% confidence limits for warm and cold thresholds, through methods of limits and levels as well as inter-session repeatability are given. Penile thermal thresholds are repeatable and can be used as a valid diagnostic tool to assess somatic small fiber function in patients with lower urinary tract disorders.

Penile Thermal Sensation

To our knowledge no direct measurement of autonomic failure in the penis is currently available. Indirect techniques in clinical use, such as bulbocavernosus reflex, genital somatosensory evoked potentials and biothesiometry, all rely on large nerve fiber function. Since micturition and potency depend on small fibers of the peripheral system, tests of these fibers might be more relevant in the clinical diagnostic evaluation. We provide upper normal limits and demonstrate repeatability of small fiber mediated sensations of warm and cold sensory thresholds on penile skin.Penile warm and cold sensory thresholds were measured in 35 healthy volunteers (at 2 sessions in 27) via 2 methods.Upper normal values, expressed as 95% confidence limits for warm and cold thresholds, through methods of limits and levels as well as inter-session repeatability are given.Penile thermal thresholds are repeatable and can be used as a valid diagnostic tool to assess somatic small fiber function in patients with lower urinary tract disorders.

Possible modes of microtubule nucleation and of broadening of cortical microtubular ribbons in the ciliate Climacostomum virens

Summary In ciliates, the dense material which surrounds the proximal part of the kinetosomes (Ks) is generally considered as an MTOC which nucleates the microtubules (Mts) whose arrangement gives rise to nematodesmata, transverse (T) and postciliary (Pc) fibers. These last two ribbons are composed of a few Mts near their origin, but can widen out while extending away from the Ks. This broadening is studied for T and Pc fibers in different parts of the cell of C. virens . It is shown that, in somatic dikinetids, T and Pc fibers enlarge by progressive addition of new Mts, one after the other, as they extend far from the Ks. In membranelles and peristomial kinetids, posterior Pc fibers also enlarge by this progressive broadening . In paroral dikinetids, the posterior Pc fiber does not enlarge by this process; two other ready-made ribbons (R1, R2) are added successively to the Pc ribbon to form a wide cytopharyngeal ribbon, a process called sequential broadening . Nucleation of Pc, T, R1 and R2 microtubules is then discussed. For none of these Mts is a distinct capping dense material identified that would act as a capping MTOC. So, we hypothesize that these Mts are generated by lateral nucleation, according to two possible processes that could be combined: one is a nucleation from lateral dense material, the second from a lateral interaction with a pre-existent adjacent Mt (“linker nucleation”).

Nitric oxide synthase I (NOS-I) is deficient in the sarcolemma of striated muscle fibers in patients with Duchenne muscular dystrophy, suggesting an association with dystrophin

Previously, we have demonstrated the expression of the brain-type nitric oxide synthase (NOS-I) in the sarcolemmal region of somatic and visceral striated muscle fibers in a variety of mammalian species through the use of enzyme histochemical and immunochemical techniques. Here we report that NOS-I protein and its NADPH diaphorase (NADPHd) activity are co-localized in the sarcolemma of human skeletal muscles. NOS-I immunolabeling and NADPHd activity showed no significant variation between type I and II fibers. In muscle biopsy specimens from patients with Duchenne muscular dystrophy (DMD), both NOS-I protein and activity were absent or markedly reduced. We, therefore, propose that NOS-I is complexed with dystrophin and/or dystrophin-associated proteins, adding a novel member to the sarcolemmal dystrophin-glycoprotein complex (DGC). The nature of the NOS-I-DGC link, and its role in skeletal muscle physiology and pathophysiology remain to be elucidated.

Morphometric investigations of the connections between the posterior C-1 root and the accessory nerve in the human cranial cervical region.

The anastomosis of the spinal section of the accessory nerve and the posterior C-1 root was examined in eight human corpses at the light-microscopical level by image analysis. The numbers of nerve fibers caudal and cranial to the anastomosis were compared. The morphometric data suggest that somatic efferent fibers of the spinal accessory nerve extend to proprioceptors in the sternocleido-mastoideus and trapezius muscles.

Neural control of penile erection in the rat

The role of autonomic and somatic neural pathways involved in the control of penile erectile tissue was investigated in an in vivo rat model. Intracavernous pressure (ICP) changes were recorded during single or combined electrical stimulation of peripheral nerves in anesthetized rats. Stimulation of the pelvic and cavernous nerves elicited similar ICP increases. Ganglionic blockade abolished the response to pelvic nerve stimulation. Stimulation of the hypogastric nerve, the sensory or motor branches of the pudendal nerve, or the paravertebral sympathetic chain at L4-L5 by themselves did not produce any change in ICP. Stimulation of some of these nerves caused changes in ICP when combined with cavernous nerve stimulation. Stimulation of the paravertebral sympathetic chain reduced the ICP increase elicited by cavernous nerve stimulation. A decrease in ICP in response to cavernous nerve stimulation was also elicited by stimulation of the peripheral cut end of the sensory branch of the pudendal nerve or in paralyzed rats, the motor branch of the pudendal nerve. After sectioning the two branches of the pudendal nerve, stimulation of the sympathetic chain still reduced the ICP increase in response to cavernous nerve stimulation. Stimulation of the motor branch ofthe pudendal nerve during erection elicited by cavernous nerve stimulation was responsible for an additional ICP increase, which reached suprasystolic values. The present study confirms a proerectile role for parasympathetic pathways. Sympathetic fibers conveyed in both branches of the pudendal nerve exert an antierectile role in the rat. We idenfied an antierectile sympathetic outflow, originating in the caudal sympathetic chain, the anatomical arrangement of which remains unknown. In this model, penile erection appeared to be dependent on the recruitment of sacral parasympathetic outflow. Additional recruitment of efferent somatic fibers present in the motor branch of the pudendal nerve could participate in more rigid erection. This study provides new information about the organization of the pathways through which the rat penis is innervated, and would be of interest to investigators in the field of male sexual function.

Species-independent expression of nitric oxide synthase in the sarcolemma region of visceral and somatic striated muscle fibers

The expression and distribution of nitric oxide synthase (NOS) was studied by use of the newly designed specific histochemical NADPH diaphorase staining method and the indirect immunofluorescence technique employing an antiserum to brain NOS in visceral and somatic striated muscles of several mammalian species. Histochemical activity and immunoreactivity were located in the sarcolemma region of type I and II fibers of all muscles investigated. Visceral muscles were more strongly stained than somatic muscles. Furthermore, type II fibers, identified by staining of myosin adenosine triphosphatase activity after pre-incubation at alkaline pH, were more intensely labeled than type I fibers. In addition, NOS activity was detected in the area of the sarcolemma of intrafusal fibers. No obvious differences between species were observed. It was concluded that NOS of striated muscles probably makes up the richest and most important nitric oxide source in mammals.

Species-independent expression of nitric oxide synthase in the sarcolemma region of visceral and somatic striated muscle fibers

Species-independent expression of nitric oxide synthase in the sarcolemma region of visceral and somatic striated muscle fibers

Microscopic anatomy of pycnogonida: I. Cuticle, epidermis, and muscle

The integument of Pycnogonida (Arthropoda) consists of an epicuticle decorated with tubercles and a filamentous coat, an exocuticle with a small number of ill-defined layers, and an endocuticle whose numerous layers are composed of conspicuously cross-banded fibrils. This cuticular periodicity, attributable to cross-linked chitin, has been observed previously in uncalcified and untanned cuticle of many lower crustaceans, especially branchiopods and copepods, and in scattered examples of thin respiratory or excretory cuticles of other arthropods. It is uniformly present in all representatives of all nine pycnogonid families examined to date. Stomodeal, proctodeal, and arthrodial cuticles are devoid of the endocuticular periodicity. The cuticle is decorated with sensory filaments and setae, but is more noteworthy for a dense coverage by glands, up to 1,400/mm2. Myocuticular junctions have desmosomal fine structure previously found only in chelicerates. Muscle fine structure is that of slow fibers with long sarcomeres and a high actin to myosin filament ratio, except for cardiac muscle, which has short sarcomeres. Among the arthropods, only merostomates resemble the pycnogonids in the lack of fast somatic muscle fibers. Pycnogonids display a hybrid array of fine structural features that variously serve to relate them to some arthropod subphyla and distance them from others.

Quantitative analysis of central terminal projections of visceral and somatic unmyelinated (C) primary afferent fibers in the guinea pig.

In guinea pigs, intracellular labeling of the dorsal root ganglion (DRG) cells with Phaseolus vulgaris-leucoagglutinin (PHA-L) was used to demonstrate the central projections of somatic and visceral afferent C-fibers. The terminations of the afferent fibers were analyzed qualitatively and quantitatively with the aid of camera lucida drawings. Terminal branches of C-fibers of both somatic and visceral origin were, in general, distributed in accord with the organization of the neuropil in lamina of the spinal cord. Terminal boutons arranged from longitudinally coursing fibers were distributed in lamina I, while boutons in lamina II were scattered in an apparent random fashion. The synaptic enlargements were counted in gray matter of the spinal dorsal horn and measured on each terminal branch of a fiber. All synaptic boutons (over one thousand) of somatic fibers were found in the superficial dorsal horn (laminae I and II). More than 60% of the synaptic enlargements of the visceral afferents also were localized superficially (lamina I and adjacent dorsal funiculus) while 10-20% of the visceral enlargements appeared in deeper layers of the spinal cord. Boutons of somatic C-fibers were larger than those of visceral origin. Quantitative data of the unmyelinated afferent fibers are discussed in the context of the sensory functions of myelinated afferent fibers.

Stomatogenesis in the ditransversal ciliate Homalozoon vermiculare (Ciliophora, Rhabdophora)

H. vermiculare possesses about 10 somatic kineties on the right lateral side, 3 somatic kineties on the left lateral side and a single circumoral kinety. The somatic kineties are composed of monokinetids exc\~ept for the anterior ends of the brush kineties which are composed of dikinetids. The circumoral kinety consists of paired kinetosomes one of which is nonciliated and associated with a microtubular lamella and a nematodesma. Stomatogenesis commences when the anteriormost somatic kinetosomes in the opisthe are transformed into the nonciliated kinetosomes of the future oral dikinetid. They lose the somatic infraciliary fibers and the ciliary shaft and each gives rise to a nematodesma and a microtubular ribbon. Adjacent to each of the transformed somatic kinetosomes, a new kinetosome is assembled, thus producing an oral dikinetid anlage. The new anterior kinetosome bears a cilium and becomes the ciliferous kinetosome of the oral wreath of cilia. In protargol stained specimens, proliferation of kinetosomes can first be observed in the left lateral kineties but, eventually, each of the somatic kineties produces one kinetofragment. Thus, H. vermiculare has a holotelokinetal type of stomatogenesis. The cirumoral kinety arises from a counter-clockwise rotation (as viewed from outside the cell) of all 15 dikinetid kinetofragments and subsequent "head to tail" fusion of the fragments after cell division has been completed. The oral apparatus of the proter seems to be largely conserved during division. Some aspects of the evolution of the oral apparatus and the origin of the oral microtubular ribbons are discussed.