546 



COMPARATIVE ANATOMY 



marily, the thickening of the striate body is the anterior continuation of 

 that of the thalamus. As a result of the local enlargement of each, the 

 two become separated by a deep groove until the end of the third month. 

 The growth and elongation of the hemisphere carries the corpus 

 striatum back over the thalamus, so that it rests on the thalamus like a 

 saddle on a horse; and the two merge into what is structurally and func- 



notochord . mandibular arch 



dorsal aorta \ / Seessell's pocket 



pharynx ^ 

 thyrc-gloGsal duct 



Rathke's pocket 



tuberculura 

 posterius 

 infundibulum 



ir.esonephi os 

 soinite 



amnion { 



Pig. 451. — Diagram of median longitudinal section of four-day chick. Due to a 

 slight bend in the embryo the section is para-sagittal in the mid-dorsal region but for 

 the most part it passes through the embryo in the sagittal plane. (From Patten's 

 "Embryology of the Chick.") 



tionally a unified organ which becomes a pathway for fibers to and from 

 the cerebral cortex. In the corpus striatum these fibers form the internal 

 capsule which divides the corpus striatum into a median caudate nucleus 

 and a lateral lenticular nucleus. At the end of the fourth month, the 

 relations resemble those of the adult brain. 



Olfactory Lobes. The olfactory lobes grow out from the base of the 

 hemispheres. At first they are hollow, but as they become differentiated 

 into bulb and tract, the cavity is lost. Fibrous connexion with that part 

 of the pallium which becomes the hippocampus is effected by neurites 

 from cells in the bulb. 



