120 



THE FROG. 



straightened out, and it also recedes some distance from the 

 anterior end of the head. 



The straightening of the brain, or rectification of the cranial 

 flexure as it is sometimes termed, is apparent rather than real, 

 and is brought about principally by the formation of the cerebral 

 hemispheres (Figs. 64, 65, BC), which grow forwards from the 



BM 



PM 



DS 



PT 



RT 



TH 



TP 



FIG. 6i. Sagittal section of the head end of a Tadpole just 

 before the opening of the mouth. 



A, dorsal aorta. BC, vesicle of the hemispheres. BH, hind-brain. BM, mid-brain. 

 CH, notochord. DS, septum separating stomatodaeum and pharynx. IN, iniundibuluin. 

 P!N", pineal body. PT, pituitary body. B,S, sinus venosus. B,T, truncus arteriosus. 

 B,V, ventricle. TH, thyroid body." TI, intestine. TP, pharynx. TO, plug of 

 epithelial cells blocking up the oesophagus. "W, liver. "WGr, gall-bladder. 



fore-brain, and speedily attain so large a size relatively to the 

 other parts of the brain as to alter the direction of the axis of the 

 brain as a whole, and to completely obscure the original flexure, 

 which really persists throughout life. The receding of the 

 brain from the anterior end of the head is due to the more 

 rapid growth of the surrounding parts, and more especially of 



