THE BRAIN. 



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 oblongata, becomes of considerable interest when it is 



BL 



3? 



TH 



TP 



AN- 



FlG. 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 line. ( Cf. Fig. 113.) x 20. 



A, dorsal aorta. AN", head fold of the amniou. AW, tail fold of the amnion. AV, 

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

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

 of the cerebral hemisphere. CH, notochord. DP, proctodteal pit. G-H, hind-gut, 

 or posterior portion of the meseuteron. IN, infumlibulum. LG. lunir. NS, central 

 canal of spinal cord. O, mouth. PN, pineal body. PT. pituitary body. RS, sinus 

 venosus. B.T, truncus arteriosus. B,V. ventricle. SP. splanclmopleure. TA, 

 allautois. TH, thyroid body. TL, tail. TO, oesophagus. TP, jiharyux. 



