120 



THE FROG. 



straightened out, and it also recedes some distance from the 

 anterior end of the head. 



T he strai p'hteninf y of the bra in. orjsdificatkm-Qf-tlifi-JMaBial 

 flexure as i t is s ometimes terme d, is apparen t rather than rea l, 

 and is brought about pri ncipally by the formation of' the cerebral. 

 hemispheres (i^'igs. 64, 65, bc), which grow forwards from the 



-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, hiud-brain. BM, mid-brain. 

 CH, notochord. DS, septum separating stomatodEeumaudpharynx. TN, infundibulum. 

 PM", pineal body. PT, pituitary body. RS, sinus venosus. RT, truncus arteriosus!. 

 RV, ventricle. TH, thyroid body. TI, intestine. TP, pharynx. TOj plug of 

 epithelial cells blocking up the cesophagus. "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 



