THE BRAIN. 513 



the optic stalks are more markedly constricted than before ; and 

 the optic vesicles, now doubled up to form the optic cups, are 

 smaller relatively to the other parts of the brain than at the 

 earlier stages. 



The mid-brain, BM, still remains small ; it is connected with 

 the hind-brain by a rather long and narrow neck. 



The hind-brain is very wide anteriorly; the cerebellum is 

 much more conspicuous than before, and consists of two lateral 

 ridges, separated by a median notch. The sides of the medulla 

 oblongata are thick, and the roof extremely thin. 



The fifth week. During the fifth week (c/. Fig. 205) the 

 cerebral hemispheres increase rapidly, growing backwards along 



/BM 

 BF, 



BL 



NS 



BY 



FIG. 217. The brain of a Human Embryo lettered by Professor His, Zw, and 

 estimated as about the middle of the eighth week. (From His.) x 5. 



BF, thalamencephalon. BH, medulla oblongata. BL, cerebellum. BM, mid- 

 braiu. BS, cerebral hemisphere. B Y, olf actor v lobe. NS, spinal cord. OS, optic- 

 stalk. 



the sides of the thalamencephalon; from the ventral surfaces 

 of their anterior ends the olfactory lobes arise as hollow out- 

 growths. The infundibulum remains of great depth ; and the 

 mid-brain is relatively smaller than before. In the hind-brain 

 the cerebellum, has increased in size ; and both the meten- 

 cephalic and the cervical flexures have increased in sharpness. 



The sixth, to the eighth weeks. The most marked change 

 during the latter part of the second month consists in the great 

 increase in the sharpness of the metencephalic flexure, which 

 amounts to nearly 180 (Fig. 217), the cerebellum and the roof 



L L 



