638 



DEVELOPMENT OF THE EMBRYO. 



236. 



cavities communicate freely with each other, through the interven- 

 ing constrictions. 



Very soon the anterior and the posterior cerebral vesicles suffer 

 a further division ; the middle one remain- 

 ing undivided. The anterior vesicle thus 

 separates into two portions, of which the 

 first, or larger, constitutes the hemispheres, 

 while the second, or smaller, becomes the 

 optic thalami. The third vesicle also sepa- 

 rates into two portions, of which the ante* 

 rior becomes the cerebellum, and the pos- 

 terior the medulla oblongata. 



There are, therefore, at this time five 

 cerebral vesicles, all of whose cavities com- 

 municate with each other and with the 

 central cavity of the spinal cord. The 

 entire cerebro-spinal axis, at the same time, 

 becomes very strongly curved in an ante- 

 rior direction, corresponding with the ante- 

 rior curvature of the body of the embryo 

 (Fig. 237); so that the middle vesicle, or 

 that of the tubercula quadrigemina, occu- 

 pies a prominent angle at the upper part of 



the encephalon, while the hemispheres and the medulla oblongata 

 are situated below it, anteriorly and posteriorly. 



At first, it will be observed, the relative size of the various parts 

 of the encephalon is very different from that which 

 they afterward attain in the adult condition. The 

 hemispheres, for example, are hardly larger than 

 the tubercula quadrigemina ; and the cerebellum 

 is very much inferior in size to the medulla oblon- 

 gata. Soon afterward, the relative position and size 

 of the parts begin to alter. The hemispheres and 

 tubercula quadrigemina grow faster than the poste- 

 rior portions of the encephalon ; and the cerebellum 

 becomes doubled backward over the medulla oblon- 

 gata. (Fig. 238.) Subsequently, the hemispheres 

 rapidly enlarge, growing upward and backward, 

 so as to cover in and conceal both the optic tha- 

 lami and the tubercula quadrigemina (Fig. 239); the cerebellum 

 tending in the same way to grow backward, and projecting farther 



Formation of the CEREBRO- 

 SPINAL Axis. 1. Vesicle of 

 the hemispheres. 2. Vesicle of 

 the tubercala qnadrigemina. 3. 

 Vesicle of the medulla oblongata. 



Fig. 237. 



* 



FOETAL Pio, five- 

 eighths of au inch 

 long, showing brain 

 and spinal cord. 1. 

 Hemispheres. 2. Tu- 

 bercu'.a quadrigemi- 

 na. 3. Cerebellum. 

 4. Medulla oblongata. 



