Revision as of 12:53, 22 October 2016

Embryology - 19 Apr 2024 Expand to Translate
Google Translate - select your language from the list shown below (this will open a new external page)
العربية \| català \| 中文 \| 中國傳統的 \| français \| Deutsche \| עִברִית \| हिंदी \| bahasa Indonesia \| italiano \| 日本語 \| 한국어 \| မြန်မာ \| Pilipino \| Polskie \| português \| ਪੰਜਾਬੀ ਦੇ \| Română \| русский \| Español \| Swahili \| Svensk \| ไทย \| Türkçe \| اردو \| ייִדיש \| Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations)

Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Chapter XIII The Central Nervous System

I. Histogenesis Of The Nervous Tissues

The nervous tissues and sensory epithelia are derived from portions of the primitive integument. The anlage of the entire nervous system is a thickened band of ectoderm along the mid-dorsal line of the embryo. This is the neural plate (Figs. 57 and 58), which, in embryos of 2 mm., develops a deep neural groove, bounded laterally by paired neural folds (Fig. 236 A-C). The folds presently meet and fuse, thereby forming the neural tube (D) which lies below the surface of the general ectoderm and becomes separated from it (Fig. 241). The cells of this tube, and its associtaed ganglion crests, give rise to all the nervous tissues, with the single exception of the nerve cells and fibers of the olfactory epithelium.

Fig. 236. - Sections of the developing neural tube in human embryos (adapted by Prentiss). A, An early stage; B, 2 mm.; C, 2 mm.; D, 2.7 mm.

The cells of the neural tube differentiate into two products. These are nerve cells, in which irritability and conductivity have become the predominant functions, and neuroglia cells, which constitute the distinctive supporting tissue of the nervous system. The wall of the neural tube, consisting at first of a single layer of columnar cells (Fig. 237 A), becomes many-layered; the com]jonent cells lose their sharp outlines and form a comijact syncytium which is bounded, on its outer and inner surfaces, by an external and lutcrual limiting membrane (B, C ). In lo mm. embrvos, the cellular strands of the syncytium are arranged radially and nearly parallel (D). The nuclei are now so grouped that there may be distinguished three layers: (i) an inner ependymal zone, with cells abutting on the internal limiting membrane and their processes extending peripherally; (2) a middle, nucleated mantle zone; and (3) an outer, non-cellular marginal zone, into which nerve fibers grow. The ependymal zone contributes cells for the development of the mantle layer (A~D). The cellular mantle layer forms the gray substance of the central nervous system, while the fibrous marginal layer constitutes the white substance.

Fig. 237. - The differentiation of the neural tube (Hardesty). X 690. . 4 , From a rabbit embryo before the closure of the neural tube; B, from a 5 mm. pig embryo after closure; C, from a 7 mm. pig embryo; /t, from a 10 mm. pig embryo. *, Boundary between mantle and marginal layers.

The primitive germinal cells of the neural tube divide by mitosis and give rise to the ependymal cells of the ependymal zone and to indifferent cells of the mantle layer (Fig. 238). From these latter differentiate spongioblasts and neuroblasts. The spongioblasts transform into neuroglia cells Sind fibers, which become the supporting tissue of the central nervous system ; the neuroblasts are primitive nerve cells, which develop cell processes and are converted into neurons. A neuron is the structural and functional unit of nervous tissue.

The Differentiation of Neuroblasts. - The nerve fibers develop as outgrowths from the neuroblasts, and a nerve cell with all its processes constitutes a neuron. The origin of the nerve fibers as processes of the neuroblasts is seen best in the development of the root fibers of the spinal nerves.

Fig. 238. - Diagrams showing the differentiation of the cells of the neural tube (after Schaper).

The Development of Efferent Neurons. - At the end of the first month, clusters of neuroblasts separate from the general syncytium in the mantle layer of the neural tube. The neuroblasts become pear-shaped, and from the small end of the cell a slender, primary process grows out (Fig. 239 A, - F'\ B). This process is the axon, or axis cylinder. Such primary processes may course in the marginal layer of the neural tube (Fig. 239 A, a), or, converging, may penetrate the marginal layer ventro-laterally and form the ventral roots of the spinal nerves (Fig. 240). Similarly, the efferent fibers of the cerebral nerves grow out from neuroblasts of the brain wall. Within the cytoplasm of young nerve cells and their primary processes, strands of fine fibrils occur (Fig. 239 A). These, the ncurofibrillcc, are usually assumed to be the conducting elements of the neurons. The cell bodies of the efferent neurons soon become multipolar by the development of branched secondary processes, the dendrons or dendrites.

Fig. 239. - The differentiation of neuroblasts in chick embryos of the third day (Cajal). A, Transverse section through the spinal cord, showing axons (F) growing from neuroblasts into the ventral root, and from bipolar ganglion cells (i/) into the dorsal root. B Single neuroblasts, showing neurofibrils and growing tip(*).

Fig. 240. - Transverse section of the spinal cord from a human embryo of five weeks, showing the origin of ventral root fibers from neuroblasts (His). X 130.

Development of the Spinal Ganglia and Afferent Neurons

After the formation of the neural plate and groove, a longitudinal ridge of cells appears on each side where the ectoderm and neural plate join (Fig. 241 A). This ridge of ectodermal cells is the neural, or ganglion crest. When the neural tube closes and the ectoderm separates from it, the cells of the ganglion crest overlie the neural tube dorso-laterally (Fig. 241 B, C). As development continues they separate into right and left linear crests, distinct from the neural tube, and migrate ventro-laterally to a position between the neural tube and myotomes (Fig. 212). In this position, the ganglion crest forms a band of cells extending the whole length of the spinal cord and as far cephalad as the otic vesicles. At regular intervals in its course along the spinal cord, the proliferating cells of the crest give _-,i. - /,â– , rise to enlargements, the spinal ganglia (Figs. 278 and 279). The spinal a -/V ganglia are arranged segmentally and are connected at first by cellular bridges that later disappear. In the hind-brain region, certain ganglia â€¢ of the cranial nerves develop also from the crest but are not segmentally arranged.

Fig. 241. - The development of the ganglion crest in a 2.5 mm. human embryo (after Lenhossek).

The cells of the spinal ganglia differentiate into ganglion cells and supporting cells, groups which are comparable to the neuroblasts and spongioblasts of the neural tube. The neuroblasts of the ganglia become fusiform and develop a primary process at each pole; thus, these neurons are of the bipolar type (Fig. 23Q A, d). The centrally directed processes of the ganglion cells converge, and, by elongation, form the dorsal roots. They penetrate the dorso-lateral wall of the neural tube, bifurcate, and course cranially and caudally in the marginal layer of the spinal cord. By means of branched processes they come in contact with the neurons of the mantle layer. The peripheral processes of the ganglion cells, as the dorsal spinal roots, join the ventral roots, and with them constitute the trunks of the spinal nerves (Fig. 246) the bifurcation of a single process (C). The process of the unipolar ganglion is then T-sha])ed (B)- Many of the bipolar ganglion cells persist in the adult, while others develop several secondary processes and thus become multipolar in form. In addition to forming the spinal and cranial ganglion cells, neuroblasts of the ganglion crest are believed to migrate ventrally and form the sympathetic ganglia (Fig. 246).

At first bijjolar (Fig. 242, A), the majority of the ganglion cells become unipolar, either by the fusion of the two primary processes or by .

Fig. 242. - Stages in the formation of unipolar ganglion cells (Cajal). From a human fetus of ten weeks.

Differentiation of the Supporting Elements. In the Neural Tube

The si)ongioblasts of the neural tube differentiate into the supporting tissue of the central nervous system. This includes the ependymal cells, which line the neural cavity and constitute one of the primary layers of the neural tube, and neuroglia cells and their fibers.

A preceding paragraph describes how the strands of the syncytium, formed by the spongioblasts, become arranged radially in the neural tube of early embryos (Fig. 237 D). As the wall thickens, the strands elongate equally and form a radiating, branched framework (Fig. 243). The group of spongioblasts which lines the neural cavity constitutes the ependymal layer. Processes from these cells extend through the neural tube, even to its periphery. The cell bodies are columnar and persist as the lining of the central cavities of the spinal cord and brain (Fig. 244).

Near the midplane of the adult spinal cord, both dorsally and ventrally, the supporting tissue retains its primitive ependymal structure (Fig. 244). Elsewhere, the supporting framework is differentiated into neuroglia cells and fibers. The neuroglia cells form part of the spongioblastic syncytium and are scattered through the mantle and marginal layers of the neural tube. By proliferation they increase in number, and their form depends upon the pressure of the nerve cells and fibers which develop around them. Neuroglia fibers are differentiated in a manner comparable to that of connective-tissue fibers (Fig. 203). As the cytoplasmic processes of the neuroglia cells primarily form a syncytium, the fibers may extend from cell to cell. The neuroglia fibers develop late in fetal life and undergo a chemical transformation into neurokeratin, the same substance that is found in the sheaths of myelinated fibers.

Fig. 243. - Ependymal cells from the neural tube of a chick (Cajal). . 4 , Embryo of first day; B, of third day.

Supporting Elements of the Ganglia

The supporting cells of thespinal ganglia at first form a syncytium, in the meshes of which are found the neuroblasts. They differentiate into flattened capsule eells, which encapsulate the ganglion cells, and into sheath cells, which envelop the axon processes of both dorsal and ventral root fibers and are continuous with the capsules of the ganglion. It is certain that many of the sheath cells migrate peripherally along with the developing nerve fibers. They are .

Fig. 244. - Ependymal cells of the spinal cord, from a fetus of ten weeks (Cajal). /I, Floor plate; B, central canal; C, line of future fusion of neural walls; E, ependymal cells; *, neuroglia cells and fibers.

at first spindle-shaped, and, as primary sheaths, enclose bundles of nerve fibers. Later, by the proliferation of the sheath cells, the bundles are separated into single fibers, each with its sheath of Schwann, or neurilemma. Every sheath cell forms a segment of the neurilemma, the limits of contiguous cells being indicated by constrictions, the nodes of Ranvier.

The Myelin Sheath. - -During the fourth month an inner myelin, or medullary sheath appears about many nerve fibers. This consists of a spongy framework of neurokeratin in the interstices of which a fatty substance, myelin, is deposited. The origin of the myelin sheath is in doubt.

By some ( Ranvier) it is believed to be a differentiation of the neurilemima, the myelin being deposited in the substance of the nucleated sheath cell. Others regard the myelin as a direct product of the axis cylinder (Kolliker), or as an intercellular substance precipitated through its influence (Bardeen). The integrity of myelin is dependent at least upon the nerve cell and axis cylinder, for, when a nerve is cut, it very soon shows degenerative changes. In the central nervous system there is no distinct neurilemma sheath investing the fibers. However, sheath cells are said to be present and most numerous during the period when myelin is developed. Hardesty traces their origin to the spongioblastic supporting cells of the neural tube, and believes that the myelin of the fibers arises in the interspaces.

The myelinated fibers, those with a myelin sheath, have a glistening white appearance and give the characteristic color to the white substance of the central nervous system and to the peripheral nerves. The fibers which are first functional receive their myelin sheaths first. This process is only completed during the third year of infancy. Many of the peripheral fibers, especially those of the sympathetic system, remain unmyelinated but are supplied with a neurilemma sheath. Large numbers of unmyelinated fibers occur also in the peripheral nerves and spinal cord.

The Neuron Doctrine. - The neuron concept of the development of nerve fibers is the one generally adopted at the present time. It assumes that all axons and dendrites are formed as outgrowths from nerve cells, an hypothesis first promulgated by His. The embryological evidence is supported by experiment. It has long been known from the work of Waller that if nerves are severed, the fibers distal to the point of section, and thus isolated from their nerve cells, will degenerate, but that regeneration will take place from the central stumps of cut nerves, the fibers of which are still connected with their cells. IMore recently, Harrison, experimenting on amphibian larvse, has shown that peripheral nerves do not develop if the neural tube and crest are removed, and that isolated ganglion cells growing in clotted lymph will give rise to long axon processes in the course of four or five hours.

.A second theory, supported by Schwann, Balfour, Dohm, and Bethe, but not widely credited, assumes that the nerve fibers are in part differentiated from a chain of cells, so that the neuron would represent a multicellular, not a unicellular structure. Apathy and 0. Schulze modified this cell-chain theory by assuming that the nerve fibers differentiate in a syncytium which intervenes between the neural tube and the peripheral end organs. Held further modified the theory by claiming that the proximal portion of a nerve fiber is derived from the neuroblast or ganglion cell and that this grows into a syncj-ffium which gives rise to the peripheral portion .

u. Morphogenesis of the Central Nervous System The primitive neural tube is formed by the folding of the neural plate into an epithelial tube, as described in the previous section. The groove begins to close in embryos of 2 mm. along the mid-dorsal line, near the middle of the body, and the closure advances in both directions (Fig. 245).

Until after the fourth week, however, there still persists a neuroporic opening at each end of the neural tube, somewhat dorsad (Fig. 251). But before the closure of the neuropores, even in embryos of 2.5 mm. or less, the cranial end of the neural tube has enlarged and constricted at two points to form the three primary brain vesicles. The caudal two-thirds .

Fig. 245. - Human embryo of 2.4 mm., with a ])artially closed neural tube (Kollmanni. X 30.

Of the neural tube, which remains smaller in diameter, is the anlage of the spinal cord.

The Spinal Cord

The spinal portion of the neural tube is at first nearly straight, but as the embryo flexes it also is bent into a curve, convex dorsally (Fig. 282). Its wall gradually thickens during the first month and the diameter of the central canal is diminished from side to side. By the end of the first month, three layers have been developed in the manner already described (Fig. 246). These layers are the inner ependymal layer, which forms a narrow zone about the neural cavity, the middle, cellular mantle layer, and the outer, fibrous marginal layer.

The Ependymal Layer differentiates into a dorsal roof plate and a ventral floor plate (Fig. 247). Laterally, its proliferating cells contribute .

Fig. 246. - Transverse section through a 10 mm. human embryo at the level of the arm budsshowing the spinal cord and a spinal nerve (Prentiss). X 44 .

neuroblasts and neuroglia cells to the mantle layer. This proliferation ceases first in the ventral floor, which is thus narrower than the dorsal portion in 10 to 20 mm. embryos (Figs. 246 and 247). The neural cavity is for a time somewhat rhomboidal in transverse section, wider dorsally than ventrally. Its lateral angle forms the sulcus limitans (Fig. 255), which marks the subdivision of the lateral walls of the neural tube into the donsal alar plate (sensory) and ventral basal plate (motor). When the ependymal layer ceases to contribute new cells to the mantle layer, its walls are a])proximated dorsally (Fig. 247). At about nine weeks, .

Fig. 248. - Transverse section of the s])inal cord and ganglion from a fetus of nine weeks (Prentiss). X 44.

These walls fuse and the dorsal portion of the neural cavity is obliterated (Fig. 248); in a fetus of three months, the persisting cavity is becoming rounded into the definite central canal (Fig. 249). The cells lining the central canal are ependymal cells proper. Those in the floor of the canal form the persistent floor plate. Their fibers extend ventrad, reaching the surface of the cord in the depression of the ventral median fissure.

.When the right and left walls of the ependymal layer fuse, the ependymal cells of the roof plate no longer radiate, but form a median septum (Fig. 248). Later, as the marginal layers of either side thicken and are approximated, the median septum is extended dorsally. Thus, the roof plate is converted into part of the dorsal median septum of the adult spinal cord (Fig. 249).

The Mantle Layer, as already described, receives contributions from the proliferating cells of the ependymal layer. A ventro-lateral thickening first becomes prominent in embryos of 10 to 15 mm. (Fig. 246). This is the ventral {anterior) gray colunm, which in later stages is subdivided forming also a lateral gray colmnu (Fig. 249). It is a derivative of the basal plate. In embryos of 20 mm., a dorso-lateral thickening of the mantle layer is seen, the cells of which constitute the dorsal (posterior) gray column (Figs. 248 and 249); about these cells the collaterals of the dorsal root fibers end. The cells of the dorsal gray column, derivatives of the alar plate of the cord, thus form terminal nuclei for the afferent spinal nerve fibers. Dorsal and ventral to the central canal, the mantle layer forms the dorsal and ventral gray commissures. In the ventral floor plate, nerve fibers cross from both sides of the cord as the ventral white commisstire.

Fig. 249. - Transverse section of the spinal cord from a fetus of three months (Prentiss). X 44 .

The Marginal Layer is composed primarily of a framework of neurogliaand ependymal-cell processes. Into this framework grow the axons of nerve cells, so that the thickening of the marginal layer is due to the increasing number of nerve fibers contributed to it by extrinsic ganglion cells and neuroblasts. When their myelin develops, these fibers form the white substance of the spinal cord.

The fibers have three sources (Fig. 281) ; (i) they may arise from the spinal ganglion cells, entering as dorsal root fibers and counsing cranially and caudally in the marginal layer; (2) they may arise from neuroblasts in the mantle layer of the spinal cord, (a) as fibers which connect adjacent nuclei of the cord (fasiculi propru or ground bundles), or (b) as fibers which extend upward to the brain; (3) they may arise from neuroblasts of the brain, (a) as descending tracts from the brain stem, or (6) as long, descending corticospinal tracts from the cortex of the cerebrum. Of these fiber tracts, (i) and (2 a) appear during the first month; (2 b) and (3 a) dui'ing the third month; (3 b) at the end of the fifth month.

The dorsal root fibers from the spinal ganglion cells, entering the cord dorso-laterally, subdivide the white substance of the marginal layer into dorsal and lateral funiculi (Fig. 248). The lateral funiculus is marked off by the ventral root fibers from the ventral funiculus (Fig. 246). The ventral root fibers, as we have seen, take their origin from the neuroblasts of the ventral gray column in the mantle layer. They are thus derivatives of the basal plate.

The dorsal funiculus is formed chiefly by the dorsal root fibers of the ganglion cells, and is subdivided into two distinct bundles, the fasicul us f^racilis, median in position, and the fascicul us nnicalus, lateral (Fig. 240). The dorsal funiculi are separated only by the dorsal median septum.

The lateral and ventral funiculi are composed: (i) of fasciculi propru, or ground bundles, originating in the spinal cord; (2) of ascending tracts from the cord to the brain; (3) of the descending fiber tracts from the brain. The fibers of these fasciculi intermingle and the fasciculi are thus without sharp boumlaries. The floor plate of ependymal cells lags behind in its development, and, as it is interposed between the thickening right and left walls of the ventral funiculi, these do not meet and the ventral median fissure is produced (cf. Figs. 246 and 240).

The development of myelin in the nerve fibers of the cord begins late in the fourth month of fetal life and is completed between the fifteenth and twentieth years. Myelin appears first in the root fibers of the spinal nerves and in those of the ventral commisSure, next in the ground fjundles and dorsal funiculi. The corticospinal (pyramidal) fasciculi are the tardiest; they become myelinated during the first and second years. As myelin appears in the various fiber tracts at different periods, this condition has been utilized in tracing the extent and origin of the various fasciculi in the central nervous system.

The Cervical and Lumbar Enlargements. - The spinal cord enlarges at the levels of the two nerve plexuses supplying the upper and lower extremities. As the fibers to the muscles of the extremities arise from nerve cells in the ventral gray column, the number of these cells and the mass of the gray substance is increased ; since larger numbers of fibers from the integument of the limbs also enter the cord at this level, there are likewise present more cells about which sensory Ihe brain and cord of a three- ^^ers terminate. Consequently, there is formed months - fetus (Kouiker). at the level of the origin of the nerves of the brachial plexus the cervical enlargement, and opposite the origins of the nerves of the lumbo-sacral plexus the lumbar enlargement (Fig. 250).

After the third month, the vertebral column grows faster than the spinal cord. Since the cord is anchored to the brain, the vertebrae and the associated roots and ganglia of the spinal nerves shift caudally along the cord. For this reason, the origin of the coccygeal nerves in the adult is opposite the first lumbar vertebra and the nerves course obliquely downward, nearly parallel to the spinal cord. The tip of the neural tubje is attached to the coccyx during this period of unequal growth, so its caudal portion becomes stretched into the slender, solid cord known as the filum terminale. The obliquely coursing spinal nerves, with the filum terminate, constitute the cauda equina. Traces of the original saccular termination of the neural tube in the integument are recognizable at birth.

The Brain

Primary Divisions

The neural tube in embryos of 2 to 2.5 mm. is nearly straight, but its cranial end is enlarged to form the anlage of the brain (Fig. 245). Three regions of expansion, separated by two retarded zones of apparent constriction, subdivide the brain into three primary brain vesicles - the fore-brain (prosencephalon) , mid-brain (mesencephalon) , and hind-brain (rhombencephalon).

Fig. 251. - Reconstructions of the brain of a 3.2 mm. human embryo (His-Prentiss). X about 35. . 4 , Lateral surface; B, median sagittal section.

Both the fore- and hind-brain vesicles promptly give rise to two secondary vesicles, whereas the mid-brain remains undivided. In embryos of about 3 mm. (four weeks), the fore-brain shows indication dorsally of a fold which subdivides it into the telencephalon, with its primitive cerebral hemispheres, and the dicncephalon, which bears the optic vesicles (Fig. 251). The mid-brain retains its original designation, the mesencephalon . At 7 mm. (five weeks), the neuropores have closed and the hind-brain constricts into the mctencephalon , or future region of the cerebellum and pans, and into the myclencephalon, or medulla oblongata. The further sej)aration of these vesicles may be followed easily in stages of lo mm. (six weeks) (Figs. 262 and 265), and 14 mm. (nearly seven weeks) (Fig. 253).

Fig. 252. - Reconstructions of the brain of a 7 mm. human embryo (His-Prentiss). A, Lateral surface; B, median sagittal section.

Fig. 253. - Brain of a 14 mm. human embryo in median sagittal section (His in Sobotta). I, Optic recess: 2, ridge formed by 3, the optic chiasma; 4, infundibular recess.

Cavities. - The lumen of the simple neural tube undergoes less change than the walls (Figs. 251 to 253). The cavity of the telencephalon extends into the paired hemispheres as the lateral ventricles; that of the diencephalon (and the median portion of the telencephalon) is designated the third ventricle; the narrow canal of the mesencephalon becomes the cerebral aqueduct; the lumen of the metencephalon and myelencephalon is the fourth ventricle. The latter is continuous with the central canal of the spinal cord.

Flexures. - While the several divisions of the brain are differentiating, certain flexures appear in its roof and floor, due largely to unequal growth processes. In part, these correspond to those external bendings seen in the head and neck regions of young embryos. The first, or cephalic flexure appears as a sharp bend in the mid-brain region of embryos about 3 mm. long (Figs. 61 and 251). Soon, the angle is so acute that the long axes of the fore- and hind-brains are nearly parallel (Figs. 252 and 282). Next, two other flexures become evident at about the same time. These are the cervical flexure at the junction of brain and spinal cord, and the pontine flexure in the region of the future pons (Figs. 251 and 254). The cervical, like the cephalic flexure, corresponds to a similar bend in the gross embryo (Fig. 64). It is produced by the entire head flexing ventrad. On the contrary, the ])ontine flexure is peculiar to the brain; it bends in the opposite direction to the others and involves the floor only. Eventually, the pontine flexure straightens and disappears; the cervical flexure is nearly lost, but the cephalic flexure, somewhat reduced, persists.

Fig. 254. - Brains of human embryos (after His). . 4 , 4.2 mm. (X 20); B, 7 mm. (X 16): C, 19 mm. (X 4).

The history of the flexures of the brain and the relative growth of its different regions may be followed by comparing the brains of embryos of four, five, and seven weeks (Fig. 254), 11 weeks (Fig. 269), and 14 weeks (Fig. 274). In the adjoining table are listed the primary subdivisions of the neural tube and the parts derived from them.

DERIVATIVES OF THE NEURAL TUBE .

Primary vesicles .

Subdivisions .

Derivatives .

Cavities .

.

Telencephalon .

Rhinencephalon Cerebral cortex Corpora striata Pars optica hypothalami .

Lateral ventricles Cranial portion of third ventricle .

Prosencephalon .

Dicncephalon .

Epithalamus Thalamus Metathalamus Hypothalamus Hypophysis Tuber cinereum Mammillary bodies .

Remainder of third ventricle .

Mesencephalon .

Corpora quadrigemina Tegmentum Crura cerebri .

Acjuaeductus cerebri .

Rhombencephalon .

Metencephalon .

Cerebellum Pons .

Fourth ventricle .

Myelencephalon .

Medulla oblongata

Spinal cord

Spinal cord .

Central canal.

The Myelencephalon. - The wall of the myelencephalon, like that of the spinal cord, differentiates dorsally and ventrally into roof- and floor plates, laterally into the alar- and basal plates (Fig. 255). The boundary line between the alar and basal plates is the sulcus Umitans. The myelencephalon differs from the spinal cord, however, in that its roof plate is a broad, thin, and flattened ependymal layer (Figs. 255 5 and 256). In the alar and basal plates, the marginal, mantle, and ependymal zones are differentiated as in the spinal cord (Fig. 256). Owing to the formation of the pontine flexure at the beginning of the second month, the roof plate is broadened, especially in the cranial portion of the myelencephalon, and the alar plates bulge laterally (Figs. 257 and 258 . 4 ). The cavity of the myelencephalon is thus widened from side to side, and flattened dorso-ventrally. This is most marked cranially, where, between the alar plates of the myelencephalon and metencephalon, are formed the lateral recesses of the fourth ventricle (Figs. 258 A and 275). Blood vessels grow into the ependymal roof of the myelencephalon, and, invading the lateral recesses, form there the chorioid plexus of the fourth ventricle. This plexus consists of small, finger-like folds of the ependymal layer and its vascular, mesenchymal cover. The line of attachment of the ependymal layer to the alar plate is known as the rhombic lip (Fig. 258 D); it becomes later the tcenia and obex of the fourth ventricle (B).

Fig. 255. - Transverse sections from a 10 mm. human embryo (Prentiss). X 44. . 4 , Through the upper spinal cord; B, through the lower myelencephalon.

Fig. 256. - Transverse sections through the mj^elencephalon of a 10 mm. human embryo (His). X 37. - 1 , Through the nuclei of Nn. XI and A'//; B, through the nuclei of Nn. A - and Xu.

Fig. 258. - Dorsal views of the developing cerebellum (. 4 , His; B-D, Prentiss). A, Six weeks; B, two months; C, four months; D, five months.

In early stages, the floor of the myelencephalon is furrowed transversely by the so-called rhombic grooves, six in number; the intervals between successive grooves are neuromeres (cf. Figs. 368 and 392). Some view these as evidential of a former segmentation of the head, similar to that of the trunk (p. 229). It is more probable, however, that they merely stand in relation to certain cranial nerves and hence the segmental arrangement is secondary.

The further growth of the myelencephalon is due : to the rapid formation of neuroblasts, derived from the ependymal and mantle layers; to the development of nerve fibers from these neuroblasts; and to the invasion of fibers from neuroblasts in other parts of the brain and spinal cord.

The neuroblasts of the basal plates give rise to the efferent fibers of the cranial nerves (Fig. 256). In embryos of the sixth week, they thus constitute niotor nuclei of origin for the trigeminal, abducens, facial, glossopharyngeal, vagus complex, and hypoglossal nerves - nuclei corresponding to the ventral and lateral gray colmnns of the spinal cord. The basal plate likewise produces the reticular formation, which is derived in part also from the neuroblasts of the alar plate (Fig. 257). Some axons cross as external and internal arcuate fibers and constitute a portion of the median longitudinal bundle, a fasciculus corresponding to the ventral ground bundles of the spinal cord. Other axons grow into the marginal zone of the same side and form intersegmental fiber tracts. The reticular formation is thus differentiated into a gray portion, situated in the mantle zone, and into a white portion located in the marginal zone (Fig. 257). The marginal zone is added to further by the ascending fiber tracts from the spinal cord and the descending pyramidal tracts from the brain. As in the cord, the marginal layers of each side remain distinct, separated by the cells of the floor plate.

The alar plates differentiate a httle later than the basal plates. The afferent fibers of the cranial nerves first enter the mantle layer, and, coursing upward and downward, form definite tracts (tractus solitarius; spinal tract of fifth nerve) (Fig. 257). To these are added tracts from the spinal cord, so that an inner gray- and outer white substance is formed. Soon, however, the cells of the mantle layer proliferate, migrate into the marginal zone, and surround the tracts. These neuroblasts of the alar plate form groups of cells along the terminal tracts of the afferent cranial nerves (which correspond to the dorsal root fibers of the spinal nerves) and constitute the receptive, or terminal nuclei of the fifth, seventh, eighth, ninth, and tenth cranial nerves. Caudally, the nucleus gracilis and nucleus cuneatus are developed from the alar plates as the terminal nuclei for the afferent fibers which ascend from the dorsal funculi of the spinal cord. The axons of the neuroblasts in these receptive nuclei decussate through the reticular formation, chiefly as internal arcuate fibers, and ascend to the thalamus as the median lemniscus. Still other nuclei differentiate, the axons of which connect the brain stem, cerebellum, and fore-brain. Of these, the most conspicuous is the inferior olivary nucleus (Fig. 271).

The characteristic form of the adult myeleneephalon is determined by the further growth of the above-mentioned structures. The nuclei of origin of th cranial nerves, derived from the basal plate, produce swellings in the floor of the fourth ventricle that are bounded laterally by the sulcus limitans. The terminal nuclei of the mixed and sensory cranial nerves lie lateral to this sulcus. The enlarged cuneate and gracile nuclei bound the ventricle caudally and laterally as the cuneits and clava (Fig. 275). The inferior olivary nuclei produee lateral, rounded prominences, the olives, and ventral to these are the large cortico-spinal traets, or pyramids (Fig. 271).

The Metencephalon. - The alar plates feature prominently in the differentiation of the metencephalon. Cranial to the lateral recesses of the fourth ventricle, their cells proliferate ventrally and form the numerous and relatively large nuclei of the pons (cf. Fig. 274). The axons from the cells of these nuclei mostly cross to the opposite side and become the hrachinm pontis of the cerebellum. Many cerebral fibers from the cerebral peduncles end about the cells of the pontine nuclei; others pass through the pons as fascicles of the cortico-spinal tracts.

Fig. 259. - Median sagittal sections of the metencephalon (Prentiss). A, Two months; B, at middle of fifth month.

The Cerebellum. - When the alar plates of the cranial end of the myeleneephalon are bent out laterally by the pontine flexure, their direct continuations into the metencephalic region assume a transverse position also (Fig. 258 A). During the second month, the alar plates thicken and bulge into the ventricle (Fig. 258 A). Near the midline, paired swellings indicate the anlages of the vermis, while the remaining lateral portions represent the future cerebellar hemispheres (Figs. 258 and 275).

The cerebellar anlages grow rapidly in length, so that their surfaces are folded transversely. During the third month their walls bulge outward and form on either side a convex hemisphere connected with the pons by the hrachuini pontis (Fig. 258 C). In the meantime, the anlages of the vermis have fused in the midline, producing a single structure marked by transverse fissures. The rhombic lip gives rise to the flocculus and nodulus. Between the third and fifth months the cerebellar cortex grows faster than the deeper layers, and the principal lobes and fissures are formed (Fig. 258 C, D). The hemispheres are the last to be differentiated; their fissures do not appear until the fifth month.

The wall of the neural tube remains thin both in front and behind the cerebellum; it constitutes respectively the anterior- and posterior medullary velum of the adult (Fig. 259 B). The points of attachment of the vela remain approximately fixed, while the cerebellar cortex grows enormously. As a result, the vela are folded in under the expanding cerebellum.

Fig. 260. - Transverse sections through the mesencephalon of a 10 mm. human embryo (His). A, Through the nucleus of N. IV; B, through the nucleus of .V. uI.

The anlages of the cerebellum show at first differentiation into the same three layers which are typical for the neural tube. During the second and third months, cells from the ependymal, and perhaps from the mantle layer of the rhombic lip migrate to the surface of the cerebellar cortex and give rise to the molecular and granular layers which are characteristic of the adult cerebellar cortex. The later dift'erentiation of the cortex is not completed until after birth. The cells of the granular layer become unipolar by a process of unilateral growth. The axons of Purkinje cells and those of entering afferent fibers form the deep medullary layer of the cerebellum.

Many cells of the mantle layer take no part in the development of the cerebellar cortex, but give rise to neuroglia tissue and to the internal nuclei. Of these, the dentate nucleus is seen at the end of the third month; later, its cellular layer becomes so folded as to produce characteristic convolutions. The fibers arising from its cells form the greater part of the brachium conjunctivum.

The Mesencephalon. - ^Distinct basal and alar plates can be recognized in this subdivision of the brain, and each differentiates into the three primitive layers (Fig. 260). At the end of the first month, the neuroblasts of the basal plate give rise to the axons of motor nerves - the oculomotor .

cranial in position, the trochlear caudal (Fig. 260). In addition to these nuclei of origin, the red nucleus develops; its early history is not well understood. The mantle layer is enclosed ventrally and laterally by fiber tracts which develop in the marginal zone. These include the median and lateral leninisci, and the descending tracts from the cerebral cortex which together constitute the cerebral peduncles.

The alar plates form the paired superior and inferior colliculi, jointly known as the corpora quadrigemina (Figs. 258 5 and 269). The plates thicken and neuroblasts migrate to their surfaces, forming stratified ganglionic layers comparable to the cortical layers of the cerebellum and the cerebellar nuclei. With the development of the superior and inferior colliculi the cavity of the mesencephalic region decreases in size and becomes the cerebral aqueduct.

Fig. 261. - Transverse section through the diencephalon of a 14 mm. human embryo (His). .X 29.

The Diencephalon. - The wall of the diencephalon differentiates a dorsal roof plate, and paired alar plates which include both the lateral and ventral regions (Fig. 261). It is doubtful if the basal- and floor plates of lower levels extend into the diencephalon (Kingsbury, 1922). The roof plate becomes a thin ependymal lining to the folded tela chorioidea. Blood vessels grow into the tela and form the chorioid plexus of the third ventricle (Fig. 261). At the junction of the caudal portion of the roof plate with the alar |date is an area termed the epithalamus (Fig. 253). From it the epiphysis, or pineal body, evaginates during the seventh week (Fig. 266) and later incorporates a certain amount of mesenchymal tissue (Fig. 263). The solid, conical epiphysis corresponds but partially to the pineal eye of reptiles.

Each thickened alar plate is divided by the sulcus limitans (Fig. 262) into the dorsal thalamus and metathalamus and ventral hypothalamus (Figs. 253 and 263). The metathalamus, really a part of the definitive thalamus, gives rise to the geniculate bodies. Several structures develop from the hypothalamic floor. Passing caudad, these are the infundibulum, tuber cinereum, and mammillary recess (Fig. 262). The lateral walls of the latter enlarge into paired mammillary bodies (Fig. 267).

Fig. 262. - Median sagittal section through the fore- and mid-brain of a 10 mm. human embryo (His).

Fig. 263. - Median sagittal section of the brain from a fetus of the third month (His in Sobotta).

The third ventricle lies largely in the diencephalon and is at first relatively broad. Owing to the thickening of its lateral walls, it is compressed to a narrow, vertical cleft (Fig. 270). The thalami are approximated, and often fuse; the niassa intermedia, thus formed, is encircled by the cavity of the ventricle (Fig. 273 B).

Fig. 21)4. - (Jblique section through the di- and telencephalon of a 10 mm. human embryo (Prentiss). X 61.

Fig. 265. - Lateral view of the fore- and mid-brain of a 10 mm. human emliryo (His).

The Hypophysis . - The hypophysis, or pituitary body, has a double origin. Its glandular portions develop from the ectodermal Rathke - s pouch, which appears at about 3 mm. just in front of the pharyngeal membrane (Fig. 91). This ])Ouch early comes in contact with a saclike extension of the infimdibuhun, the anlage of the neural hypophyseal lobe (Figs. 262 to 264, and 392). Rathke - s pouch, at first flat, grows laterally and caudally about the neural lobe, and loses its stalked connection with the oral epithelium at the end of the second month (Fig. 415). The original cavity of the pouch becomes the residual lumen of the adult gland. In embryos of about seven weeks, its walls differentiate into the glandular cords of the anterior lobe. That portion of the wall between the lumen and the neural lobe remains thin and constitutes the pars intermedia. Recently, a further glandular portion, the pars tnberalis, has been recognized, lying along the tuber cinereum; it develops from the fusion of paired lateral lobes, at the base and in front of Rathke - s pouch. The anlage of the neural lobe is transformed into a solid mass of neuroglia tissue which remains connected to the diencephalon by a permanent infundibular stalk (Fig. 418). The anterior lobe and the pars intermedia elaborate important internal secretions.

Fig. 266. - Dorsal surface of the fore- and mid-brain of a 14 nim. human embryo (His). The pallium of the telencephalon is cut away, exposing the lateral ventricle.

The Telencephalon. - Like the diencephalon, this specialized division of the neural tube represents, for the most part, greatly expanded alar plates. It is convenient to regard the telencephalon as consisting of a median portion, continuous with the diencephalon and containing the cranial part of the third ventricle, and of lateral hemispheric outgrowths (Fig. 266). Toward the end of the first month, each cerebral hemisphere differentiates into corpus striatum (a ventral portion continuous with the thalamus), pallium (primitive cerebral cortex), and rhinencephalon (olfac.

Fig. 268. - Transverse section through the fore-brain of a 15 mm. human embryo (His).

The Corpus Striatum . - The floor of each hemisphere produces a thickening (Fig. 252 B), which, at six weeks, bulges prominently into the lateral ventricle (Figs. 266 and 268). The corpus striatum, so formed, is in line caudally with the thalamus of the diencephalon and is closely connected with it, both developmentally and functionally. The corpus striatum elongates in company with the cerebral hemisphere, its caudal portion curving around to the tip of the inferior horn of the lateral ventricle and forming the slender tail of the caudate nucleus (Figs. 269 and 272). The thickening of the corpus striatum is due to an active proliferation of cells in the ependymal layer which give rise to a prominent mass of mantle layer cells. Nerve fibers passing in both directions between the thalamus and the cerebral cortex course through the corpus striatum as laminae which are arranged in the form of a wide V, open laterally. This V-shaped tract of white fibers is the internal capsule. Its cranial limb partly divides the corpus striatum into the caudate and lenticular nuclei; the caudal limb of the capsule extends between the lenticular nucleus and the thalamus (Fig. 270). The thalamus and corpus striatum are separated by a deep groove until the end of the third month (Fig. 268). As the structures enlarge, the groove between them disappears and they form one continuous mass (Fig. 270). According to some investigators there is direct fusion between the two.

Fig. 269. - The fetal brain at nearly three months (His). IMost of the right pallium is removed.

The Pallium. - The i)allial walls expand rapidly until they overlap and conceal much of the other brain structures (Figs. 267, 274 and 277). During this growth the median lamina between the two hemispheres lags in development, and thus there is formed the longitudinal fissure (Fig. 266). The lamina extends from the ventrally situated optic chiasma upward and backward to the roof plate of the diencephalon; it becomes the lamina terminalis, the cranial boundary of the third ventricle (Fig. 263). The lateral ventricles, or cavities of the hemispheres, at first communicate broadly with the third ventricle through the interventricular foramina (of Monro) (Fig. 264). Later, each foramen is narrowed to a slit, not by constriction, but because its boundaries grow more slowly than the rest of the telencephalon (Fig. 268).

Fig. 270. - Horizontal section through the fore-brain of a five-months - fetus (His).

The Rlunencephalon . - During the sixth week a swelling appears on the ventral surface of each cerebral hemisphere (Fig. 265). These enlarge into distinct olfactory lobes, which, however, remain small in man (Figs. 267 and 271). Each lobe includes an anterior and posterior division. The pars anterior is the anlage of the olfactory bulb and tract; the latter receives the backward-growing olfactory fibers, and the original lumen is lost. The pars posterior is a thickening of the brain wall which later constitutes the anterior perforated substance and the parolfactory area (Figs. 271 and 277).

The olfactory apparatus includes also a pallial portion. It is termed the ar chi pallium, because it forms the entire primitive wall of the cerebrum, a condition permanent in fishes and amphibia. In mammals, the neopallium, or adult cortex, becomes dominant and the archipallium is represented by the hippocampus (Figs. 266 and 269), a portion of the hippocampal gyrus (Fig. 271), and the dentate gyrus (Fig. 272). It resembles the rest of the cerebral cortex in the arrangement of its cells.

Fig. 271. - Ventral view of the brain of a three-months - fetus, to show the rhinencephalon (Kollmann).

The Chorioid Plexus of the Lateral Ventricles. - ^Just as the chorioid plexus of the third ventricle develops in the folds of the roof plate of the dieneephalon, so the thin, median wall of the pallium, at its junction with the wall of the diencephalon, is folded into each lateral ventricle. A vascular plexus, continuous with that of the third ventricle, grows into this fold, and projects into the corresponding lateral ventricle (Figs. 266 and 268). The fold of the pallial wall forms the chorioid fissure (Fig. 269), and the vascular plexus is the chorioid plexus of the lateral ventricle (Fig. 272). This is a paired structure, which, with the plexus of the third ventricle, makes a T-shaped figure, the stem of the T overlying the third ventricle and its curved arms projecting into the lateral ventricles just caudal to the interventricular foramen. Later, as the pallium expands, the chorioid plexus of the lateral ventricles and the chorioidal fissures are elongated extensively into the temporal lobes and inferior horns of the lateral ventricles (Figs. 270 and 272).

Commissures of the Telencephalon. - The important commissures are the fornix, anterior commissure, and corpus callosum. The first two are older commissures of the archipallium, while the larger corpus callosum is the great transverse bridge of the neopallium, or cerebral cortex. The commissures develop in relation to the lamina terminalis, crossing partly in its wall and partly in the fused adjacent portions of the median pallial walls. Owing to the union of the pallial walls dorsal and cranial to it, the lamina thickens rapidly during the fourth and fifth months. It is at this time that the significant development of the commissures occurs.

Fig. 272. - Transverse section through the telencephalon of a three-months - fetus (His). Th, Thalamus; Cs, corpus striatum.

The fornix takes its origin early, chiefly from cells in the hippocampus. The fibers course along the chorioidal side of the hippocampus cranially (cf. Fig. 269), passing dorsal to the foramen of Monro (Fig. 273 A). In the cranial portion of the lamina terminalis, fibers are both given off to the basal portion of the rhinencephalon and received from it. In this region, fibers crossing the midplane form the hippocampal commissure (Fig. 273 A)\ with the later growth of the corpus callosum it shifts further caudad (Fig. 272 B). Other fibers, as the diverging columns of the fornix, curve ventrad and end in the mammillary body of the hypothalamus (Fig. 273 B).

The fibers of the anterior commissure cross in the lamina terminalis, ventral to the primitive hippocampal commissure (Fig. 273 ^ 4 ). They arise in paired cranial and caudal divisions. The fibers of the former interconnect the olfactory bulbs in a horse-shoe bow. The fibers of the caudal division pass ventrally between the corpora striata and the cortex, and may be derived from one or both of these regions.

The corpus callosum appears, cranial and dorsal to the primitive hippocampal commissure, in the roof of the thickened lamina terminalis (Fig. 273 A). Through its fibers, which arise from neuroblasts in the wall of the neopallium, nearly all regions of one hemisphere are associated eventually with corresponding regions of the other. The fibers, found first in the corpus callosum, arise in the median wall of the hemispheres. AsHhe pallium expands, interstitial fibers develop which extend the corpus callosum both cranially and caudally (Fig. 273 B). In fetuses of five months, this great commissure is a conspicuous structure and shows the form which is characteristic of the adult (Figs. 272 5 and 277).

Fig. 273. - The cerebral commissures in median section (adapted by Prentiss), .1, Three months; B, four months.

The triangular interval between the fornix and corpus callosum contains a thin partition which separates the two lateral ventricles (Fig. 273 B). This scplitm pelluciditm is a membranous portion of the lamina terminalis and really is thinned, median pallial wall. As a result of stretching, caused by the growth of the corpus callosum, a cavity sometimes forms between the laminae of the septum; it is designated the space of the septum pcUncidum, or often, falsely, the^///z ventricle (Fig. 277).

Fig. 374. Lateral view of the l>rain in situ, at the middle of the fourth month (His.)

External Configuration of the Hemispheres. - The telencephalon so - expands cranially, caudally, and ventrally that four lobes may be distinguished (Fig. 274): (i) a cranial frontal lobe; (2) a dorsal parietal lobe; (3) a caudal occipital lobe; and (4) a ventro-lateral temporal lobe. The ventricle extends into each of these regions and forms respectively the anterior horn, the body, the posterior horn, and the inferior horn of the lateral ventricle.

The surface extent of the cerebral wall, the thin gray cortex, increases more rapidly than the underlying, white medullary layer. As a result the cortex is folded, producing convolutions, between which are prominent furrows, termed fissures. The chorioid fissure is formed, as already explained (p. 267), by the ingrowth of the chorioid plexus (Fig. 267). During the third month, the rhinal (Fig. 277) and hippocampal fissures develop in association with the rhinencephalon. The latter fissure represents a curved infolding along the median wall of the temporal lobe (Fig. 272); the corresponding elevation on the inner surface of the pallium is the hippocampus (Figs. 266 and 269). At the same time, the lateral fissure (of Sylvius) makes its appearance in the following way (Fig. 274): The cortex overlying the corpus striatum develops more slowly than the surrounding areas and is thus gradually overgrown by opercular folds of the frontal, parietal, and temporal lobes. The area thus covered is the insula (island of Reil), and the depression so formed is the lateral fissure (Fig. 276). These opercula are not approximated over the insula until after birth.

Fig. 275. - Dorsal view of the brain from a three months - fetus (Kollmann).

Fig. 276. - Lateral view of the right cerebral hemisphere from a seven-months - fetus (Kollmann).

Fig. 277. - Median surface of the right cerebral hemisphere from a seven-months - fetus (Kollmann).

In fetuses of six to seven months, four other neopallial depressions ; appear which later form important landmarks in the cerebral topography. ( They are: (i) the central sulcus, or fissure of Rolando, which forms the ^ dorso-lateral boundary line between the frontal and parietal lobes (Fig. '* 276); (2) the parieto-occipital fissure, which, on the median wall of the cerebrum, is the line of separation between the occipital and parietal lobes (Fig. 277); (3) the calcarine fissure, which marks the position of the â€¢ visual area of the cerebrum (Fig. 277) and internally causes the convexity termed the calcar avis; (4) the collateral fissure on the ventral surface of the temporal lobe, which produces the inward bulging on the floor of the posterior horn of the ventricle known as the collateral eminence.

Simultaneously with the development of the latter group of fissures/ appear other shallower depressions known as sulci. These have a definite arrangement, and, with the fissures, mark off from each other the various functional areas of the cerebrum. The surface convolutions between the depressions constitute the gyri of the adult cerebrum.

Histogenesis of the Cerebral Cortex . - In the wall of the pallium are differentiated the three primitive zones typical of the neural tube; the ependymal, mantle, and marginal layers. During the first two months the cortex remains thin and differentiation is slow. At eight weeks, neuroblasts migrate from the ependymal and mantle zones into the superficial marginal zone and give rise to layers of pyramidal and other cells typical of the cerebrum. The differentiation of these layers is most active during the third and fourth months, but probably continues until after birth. From the fourth month on, the cerebral wall thickens rapidly, owing to the development of fibers from the thalamus and corpus striatum, and of endogenous fibers from the neuroblasts of the cortex. The fibers form a white, inner medullary layer, surrounded by the gray cortex. Myelination begins shortly before birth, but some fibers may not acquire their sheaths until after the twentieth year. As the cerebral wall increases in thickness, the size of the lateral ventricle relatively diminishes; especially is this true of its lateral diameter.

Anomalies. - There are numerous types of defective neural tube development - most the result of arrest. These usually involve the bony investments as well, and produce conspicuous malformations.

The more or less extensive failure of the neural groove to close produces cranioschisis or rachischisis, depending on whether the region of the head or vertebral column is affected. In such instances the roof of the skull is lacking [acrania; liemicrania), or there are clefts in the vertebral canal. If the defect contains a sac-like protrusion of the membranes, the condition is known as meningocoelc; if the neural wall alone protrudes, it is cncephaloccele (brain) or myeloccele (spinal cord) ; if, as is most common, both are involved, it is meningoencepJialoccele, or meningo-myeloccele. Such a hernial condition of the spine is often called spina bifida and is most frequent in the lumbo-sacral region, where the sac may become the size of a child’s head.

An excessive fluid content in the brain cavities causes both brain and skull to enlarge, producing hydrocephaly The virtual absence of a brain is anencephaly; of the spinal cord, amyelus.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Reference