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Fig. 286.

vesicle, at an early period spring two processes, which become the optic vesicles. These ultimately develop into the retina, and other nervous parts of the eye, with the history of which the changes occurring in them will be described.

The optic vesicles are pushed downward by two large processes growing forward from the anterior vesicle (the primitive cerebral hemispheres). The anterior part of the brain then appears to be composed of two divisions, the anterior of which is subsequently developed into the cerebral hemispheres, corpora striata, and the olfactory lobes, as a whole called prosencephalon, while the hinder part, representing the anterior vesicle, receives the name of thalamencephalon. The cavity of the thalamencephalon opens behind into the

cavity of the middle cerebral vesicle, and in front communicates with the hollow rudiments

of the cerebral hemispheres, and eventually it O.....

becomes the cavity of the third ventricle. The floor of the thalamencephalon is ultimately developed into the optic chiasma, part of the optic nerves, and the infundibulum. The latter comes in contact with a process from the mouth, uniting with which it ultimately forms the pituitary body. From the posterior part of the roof of the thalamencephalon is developed the pineal gland-a peculiar outgrowth,

of unknown function, more elaborately develDiagram of the cere- oped in some of the lower vertebrates. The

brain of a chick at anterior part of the roof of the thalamenthe second day. (Ca. dial.) 1,2,3, Cere: cephalon becomes very thin, and its place is bica Vesecieles. o. Op- finally occupied by a thin membrane contain

ing a vascular plexus, which persists in the roof of the third ventricle (choroid plexus). From the sides of the thalamencephalon, which become extremely thickened, are developed the optic thalami.

The primitive cerebral hemispheres first appear as two lobes growing from the anterior part of the first cerebral vesicle. The floor of these lobes thickens, and forms the corpora striata, while

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Diagram of a vertical longitudinal section of the developing brain of a vertebrate animal,

showing the relation of the three cerebral vesicles to the different parts of the adult brain,

(Huxley.) Olf. Olfactory lobes. Fm. Foramen of Monro. Cs. Corpus striatum. Th. Optic thala

mus. Pn. Pineal gland. M.b. Mid brain. Cb. Cerebellum. Mo. Medulla oblongata. Hmp. Cerebral hemispheres Tk.E. Thalamencephalon. Py. Pituitary body. CQ. Corpora Quadrigemina C.C. Crura cerebri. PV. Pons Varolii. 1.- XII. Regions from which spring the cranial nerves. 1. Olfactory ventricle. 2. Lateral ventricle. 3. Third ventricle. 4. Fourth ventricle.

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Diagram of a horizontal section of a vertebrate brain. (Huxley.) Olf. Olfactory lobes. Lt. Lamina terminalis. Cs. Corpus striatum. Th. Optic thalamus.

Pn. Pineal gland. Mb. Mid brain. Cb. Cerebellum. Mo. Medulla oblongata. 1. Olfactory ventricle. 2. Lateral ventricle. 3. Third ventricle. 4. Fourth "ventricle. + Iter a tertio ad quartum ventriculum. FM. Foramen of Monro. 11. Optic nerves.

Fig. 289.

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the roof develops into the hemispheres proper. The cavities of these lobes become the lateral ventricles, and are connected by

the foramen of Monro, which at

the earlier periods is very wide, but subsequently narrows to a mere slit. The cerebral hemispheres are separated at an early stage by a fold of connective tissue, which ultimately forms into the falx cerebri. The hemi

spheres are greatly enof v

larged in the backward direction, so that they quite overlap the thalamencephalon and the parts developed from

the middle cerebral Of.

vesicle. The corpus callosum is subsequently formed by the fusion of the juxtaposed parts of the hemispheres.

From the anterior Chick on the third day, seen from beneath as a trans- part of the cerebral

parent object, the head being turned to one side. (Foster and Balfour.)

hemispheres arise two a'. False amnion. a. Amnion. ch. Cerebral hemis- prolongations, which phere. FB., MB., HB, Anterior Middle and Posterior cerebral vesicles. op. Optic vesicle.

develop into the olfacAuditory vesicle. ofv. Omphalo-mesenteric veins. Ht. Heart. Ao. Bulbus arteriosus. Ch. Notochord. tory bulbs; these grow Of.a. Omphalo-mesenteric arteries. vertebræ. x. Point of divergence of the splanchno- forward, and soon lose pleural folds. y. Termination of the foregut, V.

their cavities, which at first communicated with those of the ventricles.

Middle Cerebral Vesicle.—By the cranial flexure the brain is bent at the junction of the first and second cerebral vesicles;

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the first is thus turned downward, leaving the second as the most anterior part of the brain.

The upper walls of the middle cerebral vesicle are developed into the corpora quadrigemina.

The cavity of this vesicle persists as a narrow channel, forming a communication between the third ventricle in front and the fourth ventricle behind, and receives the name in the adult brain of the iter a tertio ad quartum ventriculum. The crura cerebri arise from the lower wall of this middle vesicle.

Posterior Cerebral Vesicle. - This is divided into an anterior and a posterior part. From the roof of the anterior division arises the cerebellum, and from its floor the pons Varolii.

The posterior division gives rise to the medulla oblongata.

The cavity of this vesicle is called the fourth ventricle. It is continuous with the central canal of the spinal cord. Its upper wall is thinned and forms the valve of Vieussens. It communicates with the subarachnoid space through the foramen of Magendie.

THE ALIMENTARY CANAL AND ITS APPENDAGES. When the blastoderm is bent at its anterior extremity to form the cephalic fold, it closes and forms the anterior boundary of a short canal, the upper wall of which is formed by the general blastoderm, and the lower by that part of the splanchnopleure which runs backward, leaving the somatopleure to form the pleuroperitoneal space.

It then turns forward to meet the uncleft mesoblast, forming the wall of the yolk sac,

which communicates freely with this rudimentary part of the alimentary tract.

This canal becomes closed in for a considerable extent, and is called the fore gut. It is the precursor of the pharynx, the lungs, the cesophagus, the stomach, and the duodenum. The mouth, which at this period is unformed, is developed later by an involution of the epiblast and the removal of the tissue between the fore gut and the buccal cavity.

The tail fold, in a somewhat similar manner, shuts off a canal called the hind gut, which becomes developed into the posterior part of the alimentary canal. This hind gut, until the further development of the bladder, etc., is in connection with the allantois, which arises as a bud from the lower part of the rudimentary

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hind gut.

Between these two canals an intermediate one is formed by the splanchnopleure, which, at a distance from its origin, becomes constricted, and shuts off an upper canal, the mid-gut, from the

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Alimentary canal of an embryo while the rudimentary mid-gut is still in continuity with the

yolk sac. (Kölliker, after Bischoff.) A. View from below.

B. Longitudinal Section. a. Pharyngeal plates.

a. Diverticulum of a lung. b. The pharynx.

b. Stomach. c.c. Diverticula forming the lungs. e. Liver. d. The stomach.

d. Yolk sac. f. Diverticula of the liver. 8. Membrane torn from the yolk sac.

. Hind-gut.

lower larger yolk sac, the connection between the two forming the ductus vitello-intestinalis.

Thus the primitive alimentary canal consists of an anterior and a posterior blind canal, which are closed below, and a third intermediate between these, which opens at its lower surface into the yolk sac.

As the placental circulation becomes more developed, the yolk sac shrinks and atrophies, until it is represented by a fold

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