668 REPRODUCTION. 



by transverse constrictions. They are known as the cerebral vesicles, 

 from which the different parts of the encephalon are afterward developed 

 (Fig. 218). The first or most anterior vesicle is destined to form the 

 hemispheres ; the second or middle, the tubercula quadrigemina ; the 

 third, or posterior, the medulla oblongata. All three vesicles are hol- 

 low, and their cavities communicate with each other through the inter- 

 vening orifices. 



Yery soon the anterior and posterior cerebral vesicles undergo a 

 further separation. The anterior vesicle is divided 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 



FIG. 220. 



FCETAL PIG, three centimetres long. HEAD or FCETAL PIG, nine centimetres 



1. Hemispheres. 2. Tubercula quad- long. 1. Hemispheres. 3. Cerebellum, 



rigemina. 3. Cerebellum. 4. Me- 4. Medulla oblongata. 

 dulla oblongata. 



separates into two portions, of which the anterior becomes the cerebel- 

 lum, the posterior the medulla oblongata. 



There are, therefore, at this time five cerebral vesicles, all of which 

 communicate with each other and with the central cavity of the spinal 

 cord. The entire cerebro-spinal axis also becomes strongly curved in 

 an anterior direction, corresponding with the curvature of the body of 

 the embryo (Fig. 219) ; so that the middle vesicle, or that of the tuber- 

 cula quadrigemina, occupies a prominent angle at the upper part of the 

 encephalon, while the hemispheres and the medulla oblongata are situ- 

 ated below it, anteriorly and posteriorly. The relative size of the 

 various parts of the encephalon is very different from that in the adult 

 condition. The hemispheres are hardly larger than the tubercula 

 quadrigemina; and the cerebellum is inferior in size to the medulla 

 oblongata. Soon afterward, the relative position and volume of the 

 parts begin to alter. The hemispheres and tubercula quadrigemina 

 grow faster than the posterior portions of the encephalon ; and the 

 cerebellum is doubled backward over the medulla oblongata. (Fig. 

 220.) Subsequently, the hemispheres enlarge more rapidly, growing 

 upward and backward, so as to cover both the optic thalami and the 

 tubercula quadrigemina (Fig. 221) ; the cerebellum projecting in the 

 same way over the medulla oblongata. The subsequent development 

 of the encephalon is mainly a continuation of the same process ; the 

 relative dimensions of the parts constantly changing, so that the hemi- 

 spheres become, in the adult condition (Fig. 222), the largest division 



