Human Embryology and Morphology 9

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Chapter IX. The Fore-Brain or Prosencephalon

The Origin of the Cerebrum

It is in connection with the fore-brain that the most distinctive and most complex of all human structures arises — the cerebrum. If we confine our attention purely to the developmental changes which occur in the fore-brain of the human embryo, we shaU understand very imperfectly the origin and nature of the human brain. It is true that on developmental evidence alone we may infer that the fore-brain, although situated at the anterior extremity of the neural tube, does not rejiresent a prolongation of all the elements of the tube, but only of its alar or dorsal laminae, which we know to be sensory in their nature. We may infer that the fore-brain belongs to the sensory part of the nervous system — not to its motor or basal lamina. To obtain a proper appreciation of the fore-brain, however, one must study this structure in the lowest of vertebrates — the Lamprey. In Fig. 95 the brain of this primitive fish is represented. The fore-brain is made up of two parts — a posterior — the thalamencephalon or diencephalon, with which the retinae and optic tracts are connected, and an anterior or telencephalon, in which the olfactory nerves terminate. The two parts of the fore-brain have thus arisen in connection with the sense of sight and the sense of smell ; secondary nerve masses have arisen in these two parts of the fore-brain — the optic thalamus in the posterior, and the corpus striatum in the anterior ; but the optic thalamus receives not only nerve tracts connected with the sense of sight, but other sensory tracts connecting it with all the systems of the body — skin, muscles, joints, ear, etc., and thus becomes a higher centre for the control of lower centres. The corpus striatum — ^the secondary mass in the anterior or olfactory part — the telencephalon — also receives tracts from the gustatory, and other lower centres besides those from the olfactory tracts. In the brain of the lamprey the mid-brain and the two parts of the fore-brain form a " federation of centres."[1] In mammals the telencephalon becomes the dominant part ; the cerebral hemispheres arise from it. Thus our cerebral hemispheres have arisen in connection with parts which have become insignificant — the olfactory nerve centres. The telencephalon has received and formed communications with all parts of the central nervous system, and become the central exchange of all sensory impulses and also the seat of consciousness.

Fig. 94 Longitudinal Section of the Brain of a Larval Fish, to show the primary form and relations of the fore-brain.

Fig. 94. Longitudinal Section of the Brain of a Larval Fish, to show the primary form and relations of the fore-brain. (Kupffer.) Note especially that the whole roof is formed by a choroidal velum.

Fig. 95 The Brain of the Lamprey from above.

Fig. 95. The Brain of the Lamprey from above. (After R. H. Burne.)


The Fore-Brain of the Human Embryo

In the 4th week of development there is a resemblance between the human fore-brain and that of a fish ; both are of a simple vesicular form (compare Figs. 94 and 96). In some respects the fish's brain is the more instructive, because its parts are clearly differentiated. In the fish the roof of the 3rd ventricle — the name given to the central canal of the thalamencephalon — contains no nerve tissue ; it is membranous, and forms a choroid plexus. The pineal body arises from the posterior part of the roof, immediately in front of the posterior commissure (Fig. 94). The representatives of those parts are seen in the roof of the 3rd ventricle of the human embryo (see Figs. 96, 97, 98). On the narrow floor of the 3rd ventricle are seen the infundibular part of the pituitary body and the optic chiasma — or the plate in which the chiasma will be formed. In both the fish and the; human embryo the anterior wall of the 3rd ventricle is formed by a plate of neural tissue — the lamina terminalis.


Parts Developed in the Wall of the Fore-Brain

When a model of the fore-brain of an embryo in the 4th week of development is laid open, as in Fig. 96, it is possible to identify its two main divisions — a posterior or tlialamencepiialon, its central cavity becoming the 3rd ventricle (Fig. 96, B), and an anterior or telencephalon which will enclose the lateral ventricle. At the junction of these divisions, but yet lying distinctly in the wall of the thalamencephalon, is seen the wide evagination (Fig. 96, A) which gives rise to the optic vesicle — the basis in which the retina and optic tracts will develop. A section across the thalamic region of the fore-brain at this stage shows a right and left lateral plate, their basal margins being united by a trough-like floor plate, while their dorsal margins are joined by a roof plate — which, late in the 2nd month, becomes converted into the choroid plexus of the 3rd ventricle — just as was the case with the roof plate of the 4th ventricle. The lateral plates show the usual three zones during the 4th week — an inner ej)endymal, in which cellular proliferation is active, a middle or mantle zone and an outer or marginal which, in the 3rd month, becomes invaded by the great trackways leading to and from the fore-brain. At this early stage, too, a groove can be seen running obliquely on the lateral wall of the 3rd ventricle, from the floor of the mid-brain to the optic recess (Fig. 96, B), indicating a division of the lateral plate into an upper or thalamic and a lower or hypothalamic region. In the upper region will be differentiated the optic thalamus, the epithalamus (the pineal body with its ganglia and commissures) and the metathalamus or geniculate bodies, while in the lower region and in the floor plate are differentiated the infundibular stalk of the pituitary body, the tuber cinereum, the mammillary bodies and the posterior perforated space. Here we are chiefly concerned with the walls of the 3rd ventricle, but it may be noted in Figs. 96, A and B, that the three areas of the telencephalon can also be identified — the cortical or pallial area of the cerebral evagination, the striate area — forming a junctional zone between the thalamic region of the 3rd ventricle and the pallial area of the lateral ventricle (Fig. 96, B) and an olfactory area. At the beginning of the 4th week (Fig. 96, A) the neuropore is still open, and the olfactory areas which will appear at each side of the closed opening can hardly be said at this time to be differentiated.

Fig. 96 Sections of the Fore-Brain 4th week of development

Fig. 96. Sections of the Fore-Brain at the beginning {A) and near the end (B) of the 4th week of development.

By the end of the 6th week certain notable changes have occurred in the fore-brain (Fig. 97) ; the cerebral vesicle is now rapidly expanding, its hinder or occipital end beginning to expand over and cover the roof and lateral walls of the thalamencephalon. The opening of the lateral ventricle has become relatively smaller, owing to the upgrowth and more intimate fusion of the corpus striatum with the optic thalamus. Tn the hypothalamic region we can now see a recess behind the optic chiasma, indicating the outgrowth of the infundibular process of the pituitary body (hypophysis) and growing towards it an ingrowth of ectoderm from the embryonic mouth or stomodaeum. The roof plate is now beginning to be converted into a secretory structure — ^the choroid plexus of the 3rd ventricle. The roof plate can be seen to extend (Fig. 97) from a slight dip or fold over the foramen of Monro to the region of the pineal body at the anterior border of the mid-brain.

Fig. 97 The Thalamencephalon towards the end of the 6th week of development.

Fig. 97. The Thalamencephalon towards the end of the 6th week of development.


In Fig. 98, which represents in a diagrammatic manner the simple forebrain of an embryo at the end of the first month of development, there have been represented — following a scheme devised by Professor Elliot Smith-^the great sensory pathways which terminate in the thalamencephalon and make it into the great court of sensory appeal. These fibre tracts, which do not begin to make their way through the mid-brain from the medulla and cord until the end of the 3rd month of development, are depicted by simple arrows — the medial lemniscus and auditory tract ending in the optic thalamus, while the gustatory tract ends in the hypothalamic region. Relays of fibres commencing in the thalamencephalon carry optic, auditory, gustatory and common sensory impulses to the telencephalon — the highest court of sensory appeal. It is when this broad conception of the relationship of the fore-brain to the sensory tracts is grasped that we begin to understand the reason for the transformation of the simple fore-brain of the embryo into the elaborate cerebrum of the adult.


The Lamina terminalis forms the anterior or terminal wall of the simple fore-brain of the 4th week embryo (Fig. 96, B) ; it is completed by the closure of the neuropore (Fig. 96, A). When the cerebral vesicles grow out it becomes demarcated as a plate stretching from the foramen of Monro above to the optic chiasma below (Fig. 97), and joining together the olfactory areas of the cerebral vesicles. This simple plate, which comes to form the anterior wall of the 3rd ventricle, begins to assume great importance in the second month, because it serves as a bridge for the crossing of nerve tracts between the right and left halves of the telencephalon. The development of these commissural tracts will be mentioned later ; in the meantime it may be pointed out that part of it retains almost its embryonic state in the adult and forms the lamina cinerea which closes the anterior wall of the 3rd ventricle between the oj)tic chiasma below and the corpus callosum above.

Fig. 98 Diagram of the Embryonic Fore-Brain

Fig. 98. Diagram of the Embryonic Fore-Brain, to show how its various parts become linked to sensory tracts. (Elliot Smith.)


Glands arising from the Walls of the 3rd Ventricle

We have seen that the roof plate of the 3rd ventricle is converted into a secretory structure — the choroid velum. We now proceed to note the manner in which two remarkable glandular bodies arise in connection with the 3rd ventricle — the pituitary in relationship to the anterior part of its floor and the pineal from the hindmost part of its roof. We must suppose that their functions are closely related to the nutrition and welfare of the nerve system. The position and connections of these two bodies will be seen in Figs. 99, 100. A sagittal section of the pituitary body of a foetus of the 5th month is drawn in Fig. 99, showing the neural part derived from the floor of the 3rd ventricle and the buccal, derived from the ectoderm of the primitive mouth. In the buccal part the central cavity divides the glandular mass into a part applied to the neural lobe into a paraneural or intermediate and a larger anterior part or lobe. Besides these tioo parts there is a third, the lateral or tuberal part, which is seen on section in Fig. 99, applied as a plate to the neck of the infundibulum. The pineal body of a newly born child is represented in Fig. 100 ; it is about the size of a wheat grain, resting on the roof of the mid-brain, between the superior corpora quadrigemina. On each side are seen the upper surfaces of the optic thalami.

Pituitary Body

As is so often the case in the development of the human body, procedures which take place obscurely in man, present themselves with almost diagrammatic sharpness in low vertebrates — particularly in selachians, of which the dog-fish may be taken as a type.

Fig. 99 Section of the Pituitary Body of a Human Foetus in the 5th month.

Fig. 99. Section of the Pituitary Body of a Human Foetus in the 5th month. (Edinger.)

Fig. 100  Showing the position of the Pineal Body and its commissure and ganglion.

Fig. 100. Showing the position of the Pineal Body and its commissure and ganglion.


The original saccular form of the pituitary body and its division into three parts or lobes are well seen in the pup dog-fish (Fig. 101). The original stalk is indicated, and the three parts into which the sac becomes divided by the growth and proliferation of the epithelial cells in its walls are shown. The lateral or tuberal parts arise as right and left diverticula near the root of the stalk. The tuberal part, as a distinct element of the pituitary complex, was first recognized by Dr. Tilney in 1913,[2] but since then its presence has been noted in all vertebrates, including man. The lateral or tuberal parts as they expand become applied to the infundibular region of the floor of the 3rd ventricle, their cells invading the arachnoid and occupying its meshes.

Fig. 101 Sagittal Section of the Pituitary Body of a Pup Dog-Fish.

Fig. 101. Sagittal Section of the Pituitary Body of a Pup Dog-Fish. (After Baumgartner.)

By the end of the 4th week the basis of the buccal part of the pituitary can be recognized in the roof of the primitive mouth or stomodaeuni (Fig. 102, A), just in front of the oral plate, which at this time closes the anterior end of the fore-gut. The stomodaeum is lined with ectoderm, and it is therefore an ectodermal evagination, known as Rathke's pocket, which goes to form the buccal part of the pituitary. It will be noted that the ectodermal element is closely applied to the floor of the fore-brain from the start ; in the 5th week the adjacent part of the neural floor begins to grow out, and becomes the infundibular process. One other point should be noted ; just behind the upper attachment of the oral plate the entoderm of the fore-gut forms a slight pocket. Seessel found that in some animals (birds) this pocket also took part in the formation of the pituitary, and hence is called Seessel's pocket— it, however, does not share in the production of the human pituitary.

Fig. 102 Development of the Pituitary.

Fig. 102. Development of the Pituitary. A, its condition in a Human Embryo 4 weeks old ; B, in an Embryo in the 7th week of development. (Rudel.)

By the 7th week marked changes have occurred (Fig. 102, B). The infundibular process (the neural part) is now quite evident ; its cavity is still open, becoming filled up in the 9th week. The buccal evagination has assumed a pocket form — pressing against the neural process, its neck having become drawn out into a long stalk, because the base of the skull is being laid down between the roof of the pharynx and the floor of the fore-brain. The nasal and buccal cavities are being developed, the buccal end of the stalk coming ultimately to lie at the posterior border of the nasal septum. Usually some fragments of the pituitary stalk persist in the mucous membrane on the roof of the nasopharynx ; cases occur in which, owing to a malformation of the base of the skull, the whole pituitary body lies in the posterior part of the nasal septum. By the 9th week the stalk has disappeared, but occasionally a canal in the body of the sphenoid bone of the adult — the cranio-pharyngeal canal — marks the site of the embryonic stalk.


During the 3rd month the epithelial lining of the pituitary sac grows rapidly, particularly in the anterior part where glandular masses encroach upon the lumen (Fig. 103), ultimately obliterating all but the central space between the anterior and intermediate parts. The gland encapsules itself in the tissues of the dura mater, branches of the internal carotids and mesodermal tissue entering the glandular masses as they begin to proliferate into the central cavity (Fig. 103).


Many theories have been framed to account for the position and formation of the pituitary in the floor of the 3rd ventricle, a favourite one being that it had been formed round the opening or mouth of the central canal of the nervous system when that canal was alimentary in nature. It seems more probable, judging from recent observations of Gushing, that the pituitary is so placed, because it discharges a secretion into the 3rd ventricle, which circulates in the cerebro-spinal fluid. Gaskell, who regarded the neural or cerebro-spinal canal as the homologue of the invertebrate alimentary canal, homologized the pituitary evagination of the buccal ectoderm with the invertebrate mouth and gullet, and the pituitary body itself with the coxal glands of crustaceans. The pituitary body exercises a curious influence on the growth of certain parts, especially on the face and limbs. Disease of the pituitary body may lead to overgrowth of the limbs, as in giants, or of the face, as is seen in cases of acromegaly.

Fig. 103 Coronal Section of the Pituitary Body of a Human Foetus at the beginning of the 4th month of development.

Fig. 103. Coronal Section of the Pituitary Body of a Human Foetus at the beginning of the 4th month of development. The section is across the anterior lobe.


Pineal Body or Epiphysis

In recent years it has been shown that both pituitary and pineal bodies[3] secrete substances which have a powerful influence on the development and growth of tissues, that of the pineal being more especially on those parts which are correlated with sexual maturity. The situation of the pineal body at the hinder end of the roof of the 3rd ventricle is shown in Fig. 100, but its connections — especially with the posterior commissure, habenular commissure and choroid j)lexus — are better seen in Fig. 91. Originally the Pineal organ was a complex structure, consisting, as is shown in Fig. 104, of a parietal organ or eye, the organ being socketed in the sagittal suture, and an adjacent glandular structure opening on the roof of the 3rd ventricle. In mammals, as in man, only the 3rd or glandular part, nerve nuclei and commissures are developed. In fossil reptiles and in some forms still living it forms a median eye which perforates, and appears on, the dorsum of the head, between the parietal bones. It differs from the lateral eyes which grow from the third ventricle as the optic vesicles in this, that it produces the lens as well as the retina and optic stalk. The retina is inverted — i.e. the apices of the rods and cones point towards the vitreous chamber. The ganglion of the habenula, situated on the dorsal and inner aspect of the optic thalamus, appears to represent its terminal ganglion, but it must also be remembered that this ganglion receives the striae pinealis which arise from part of the rhinencephalon. The two ganglia become connected across the roof plate by a commissure (the superior or habenular commissure) (Fig. 100).

Fig. 104 The Pineal Gland and Sense Organ in a Lizard.

Fig. 104. The Pineal Gland and Sense Organ in a Lizard. (Gaupp.)

Fig. 105 Showing stages of development of the Pineal Body in the roof of the Fore-Brain

Fig. 105. Showing stages of development of the Pineal Body in the roof of the Fore-Brain : A, in the 3rd month ; B, in the 6th month. (After Krabbe.)


The manner in which the pineal body arises in man is shown in Fig. 105. At the posterior end of the roof of the fore-brain the ependymal lining grows out as a pocket in the 6th week of development. The evaginated cells form a zone for cellular production (Fig. 105, A), as we have seen is the case everywhere in the neural tube, but in this instance the cells produced are mainly glandular in nature, there being, however, as Dr. Krabbe has observed, also some neuroglial and neuroblastic elements. From the anterior wall of the pocket a mass of cells separates early to form an anterior lobe (Fig. 105). In the sixth month (Fig. 105, B) the body is assuming its final form ; part of the recess or pocket has become closed off in the distal part of the gland. The glandular masses are invaded by vascular and mesenchymal tissue, and the same formation of interlacing columns is produced as is seen in the buccal part of the pituitary or cortical part of the adrenals. How this body has become associated with the development of the sexual system is an enigma.



  1. The phrase is Professor Elliot Smith's, whose researches on the evolution of the brain form the basis of the account given here.
  2. For recent literature relating to the development and morphology of pituitary, see Miss K. M. Parker's excellent paper, Journ. Anat. 1917, vol. 51, p. 181 (pituitary of Marsupials) ; Prof. J. E. S. Frazer, Lancet, 1916, vol. 2, p. 45 ; Dr. E. Rudel, Anat. Hefte, 1917, vol. 55, p. 187 (pituitary of Man) ; E. A. Baumgartner, Journ. Morph. 1915, vol. 26, p. 391 ; vol. 28, p. 209 ; W. J. Atwell, Anat. Eec. 1918, vol. 15, p. 73 ; Prof. P. T. Herring, Journ. Exper. Physiol. 1908, vol. 1, p. 121.
  3. Papers on development and nature of the pineal body are : Dr. Knud Krabbe, Anat. Hefte, 1916, vol. 54, p. 191 ; A. Dendy, Phil. Trans. 1911, Ser. B, vol, 201, p. 227 ; J. Warren, Amer. Journ, Anat, 1911, vol. 11, p. 313.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures