DEVELOPMENT OF THE BRAIN. 



Vom 



walls of the vesicles. Thus the corpora striata are formed in the floor of the hemi- 

 sphere vesicles, whilst the principal mass of each hemisphere is formed from the 

 roof and sides (mantle) of those vesicles, and the olfactory lobes are hollow out- 

 Fig. 68. FORE-PART OF THE EMBRYO SHOWN IN FIG. 38, 

 VIEWED FROM THE DORSAL SIDE. ~{*. (From Kb'lliker.). 

 F, fore -brain ; e, ocular vesicles ; M, mid-brain ; H, hind- 

 brain ; h, part of the heart seen bulging to the right side ; 

 Vom, omphalo-mesenteric or vitelline veins entering the 

 heart posteriorly ; Mr, medullary canal, spinal part ; 

 p, proto-vertebral somites. 



growths from them. The cavities of the hemi- 

 sphere-vesicles become the lateral ventricles, and 

 the cavity of the part of the fore-brain (or first 

 secondary vesicle) i'rom which they spring, forms 

 the anterior extremity of the third ventricle. 

 The optic thalamus is formed by a thickening 

 of the lateral wall of the second vesicle, the 

 cavity of which comes to be the main part of 

 the third ventricle ; the corpora quadrigemina 

 are thickenings in the roof, and the crura cerebri 

 thickenings of the sides and floor of the third 

 vesicle, which becomes the aqueduct of Sylvius ; 

 the cerebellum and pons are respectively thick- 

 enings of the roof and floor and the crura cere- 

 belli of the sides of the fourth vesicle (anterior 

 part of hind-brain), the cavity of which becomes 

 the anterior (superior) part of the fourth ven- 

 tricle ; and finally, the medulla oblongata is 

 developed as a thickening of the wall of the 

 fifth vesicle, the cavity of which expands from 



the central canal of the spinal cord to form the calamus scriptorius of the fourth 



ventricle. 



On the other hand, certain parts of the walls of the vesicles become thin and 



greatly expanded, and even eventually project into the cavities as folds of epithelium 



Fig. 69. OUTLINE OF A LONGITUDINAL 



SECTION THROUGH THE BRAIN OF A 



CHICK OF TEN DAYS (after Mihalkovics). 



h, cerebral hemisphere ; olf, olfactory lobe 

 and nerve ; at, corpus striatum ; lr, lateral 

 ventricle : ac,, anterior commissure ; It, lamina 

 terminalis ; ope, optic commissure ; pit, pitui- 

 tary gland ; inf, infundibulum ; cat, internal 

 carotid artery ; i^ f third ventricle ; ch 3 , cho- 

 roid plexus of third ventricle ; pin, pineal 

 gland : bg, corpora bigemina ; amv, anterior 

 medullary velum ; below which two last 

 references are the aqueduct of Sylvius and 

 crura cerebri ; cbl, cerebellum ; v 4 , fourth 



ventricle ; ba, basilar artery : ps, pons Yarolii ; ch 4 , choroid plexus of the fourth ventricle ; obi, medulla 



oblongata ; r, roof of fourth ventricle. 



covering ramified vascular expansions of pia mater (choroid plexuses). These 

 vascular expansions occur along the lower border of the mesial surface of each 

 hemisphere-vesicle (choroid plexuses of lateral ventricles) ; along the roof of the 

 second vesicle (choroid plexus of third ventricle), and in the roof of the fifth vesicle 

 (choroid plexus of fourth ventricle). 



Mi 



, 

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