474 C. JUDSON HERRICK 



wall of the diencephalon is composed extend in the primitive 

 vertebrates across the di-telencephalic boundary, as I have de- 

 scribed them, without fundamental morphological change, then 

 the differences between Johnston and the other authors as to the 

 exact position of this boundary involve no necessary change in the 

 fundamental morphology. 



On account of the very small degree of evagination of the cere- 

 bral hemisphere in cyclostomes the di-telencephalic fissure is 

 shallow and the pars dorsalis thalami passes over without inter- 

 ruption into the lateral wall (lobus olfactorius) of the hemisphere. 

 Moreover this fissure does not extend upward to the mid-dorsal 

 line and thus the dorso-median ridge is able to pass continuously 

 from one segment to the other. In higher vertebrates this fissure 

 extends dorsally up to the site of the velum transversum and it is 

 so deep as to interrupt the continuity of both the ridge and all 

 other massive tissue of the pars dorsalis thalami with their telen- 

 cephalic representatives. Intermediate conditions will piobably 

 be found in the lower fishes. In amniotic vertebrates the separ- 

 tion is made still more complete by the development of the pos- 

 terior chorioid fold of the Uemisphere. 



In the urodele (see fig. 22) most of the telencephalon except 

 the nucleus preopticus is evaginated into the hemisphere. This 

 nucleus is, however, very large. The corpus striatum complex 

 extends backward from the ventro-lateral part of the hemisphere 

 for a short distance into continuity with the pars ventralis thalami 

 and from the same part the eminentia thalami projects forward 

 into the wide interventricular foramen. The diencephalic part 

 of the dorso-median ridge is rudimentary and the telencephalic 

 part has greatly enlarged to form the primordium hippocampi. 

 We shall return to the internal morphology of the amphibian 

 brain on a later page. At this point I wish to direct attention 

 to a few features of the early development of the human brain. 



The dorso-median ridge is evident in a form very similar to that 

 of cyclostomes in the human embryo Br 3, of about four weeks, 

 figured and modeled by His. Fig. 82 is drawn from Ziegler's 

 reproduction of the His model with lettering taken from sketches 

 of the same model in the last paper published by Professor His 



