EMBRYOLOGY AND COMPARATIVE ANATOMY OF BRAIN. 21 



passes caudad in a thin lamella, the velum medullare posticum, 

 to the dorsal region of the spinal cord. The lateral portion 

 of the inter-brain, the thalamus, is not visible in the plane of 

 Fig. 11. Secondary fore-brains developed from the primary 

 fore-brain vesicle are not found in all animals. In the ray the 

 anterior wall of this vesicle simply becomes enormously thick- 

 ened, until it is a large structure which also contains the corpus 

 striatum (Fig. 12). But in many sharks we can see little swell- 

 ings on each side in front of this mass, the first rudiments of the 

 hemispheres. The fore-brain of bony fishes contains a large corpus 

 striatum, 4witjhe^dorsal portion of_this vesicle, the^ pallium, has 

 not ad vanced beyond the embryonic state of a simple layer of 



Fia. 12. 



Brain of a ray. 



Verdifikte Schlitssplatte, Thickened embryonic terminal lamina. 



epithelium. From the fish up to the human being, the corpus 

 stna'tum does not ^materially change its position .or its relative 



^_>-i-tX- rjdLr-^s^. ^1^, A/ -~J^-"^-~-v--> ? <-_. _*JUx^u>_ --- t^-v- _^ (j^^^A^ 



^ 



In the same region we find the same aggregation of gan- 

 glion-cells. In every case there arises from these cells a bundle 

 of fibres which passes backward and terminates partly in the inter- 

 brain, and in part passes farther back into the oblongata (basal 

 fore-brain bundle). The pallium, however, must pass through 

 many stages of development before there is evolved from the 

 simple epithelial layer which we have just seen in fishes that 

 massive structure which, in human beings, we call the hemi- 

 sphere. 



