THE BEAIN. 



255 



The fact that the cerebellum remains for so long a time in 

 the condition of a mere transverse thickening of the roof of the 

 medulla oblongataj becomes of considerable interest when it is 



JTiu^. 114. — A median longitudinal, or sagittal, section through a Chick Embryo 

 at the end of the third day of incubation. The amnion is represented by 

 a dotted Hne. ( Cf. Fig. 113.) x 2p. 



A, dorsal aorta. AIST, head fold of the amnion. AW, tail fold of the amnion. AV, 

 vitelline artery. BH, fourth ventricle, or cavity of medulla oblongata. BL, cerebellmn. 

 BM, cavity of mid-brain, the future Sylvian aqueduct. BS, lateral ventricle, or cavity 

 of the cerebral hemisphere. OH, notochord. DP, proctodEeal pit. GrH, hind-gut, 

 or posterior portion of the mesenteron. IW, infundibiilum. LG, lung. itS, central 

 canal of spinal oord. O, mouth. PW, pineal body. PT, pituitary body. BS, sinus 

 venosus. RT, truncus arteriosus. RV, ventricle. SP, splanchnopleure. TA, 

 allantois. TH, thyroid body. TL, tail. TO, cesophagus. TP, pharynx. 



