510 'Joiirnnl of Comparative Neurology and Psychology. 



marks have disappeared, the boundary can be defined by a line dra\vn 

 jnst behind the interventriciihir foramen and meeting the posterior 

 surface of the chiasma ridge (Fig. 44). 



The external groove which separates the diencephalon and telen- 

 cephalon is usually well marked and in the brains of amphibians, 



Fig. 43. Sketches to illustrate the boundary line between the diencephalon 

 and the telencephalon. The brains of a selachian (A) and an amphibian 

 (B) are outlined as seen from the medial surface and the boundary set by 

 His is indicated by a dotted line, that fixed in this paper by a heavy con- 

 tinuous line. 



reptiles and mammals increases in depth and prominence with the 

 enlargement of the cerebral hemispheres. The description of the 

 early development has shown that the lateral chorioid plexus is 

 formed in manunals immediately in front of the velum transversum 

 and of the groove which continues from the velum transversum 

 around the lateral wall of the brain. From this it results that in the 



