484 C. JUDSON HERRICK 



campus arises exclusively at the expense of this structure, and 

 from the Amphibia onward the gradual transformation of the 

 primordium hippocampi into cortex hippocampi can be easily 

 followed until in the mammals it is practically all so consumed. 

 In reptiles the dorso-medial cortex arises from this primordium; 

 but in old embryos there is always an unspecialized remnant of 

 it left ventrally. This sometimes is preserved in the adult; in 

 other cases it is filled with fimbria fibers and loses its identity. 

 When present, it is generally separated from the overlying cortex 

 hippocampi by a slight superficial sulcus, which is the first rudi- 

 ment of the fissura hippocampi (fissura arcuata of mammalian 

 embryology). Attention is especially called to the fact that this 

 fissure develops within the hippocampal formation and should 

 not be confused, as has often been done, with the phylogentically 

 older fissura limitans hippocampi, which separates the hippo- 

 campal formation from the underlying precommissural body 

 (see p. 464). 



There is no evidence in any vertebrate that any cells of the 

 pars dorso-medialis have been derived from the pars ventro- 

 medialis. On the contrary, these two parts appear to have 

 differentiated from prim.ordia which have retained a structural 

 independence which goes back to the primary unevaginated neural 

 tube. In this I support the opinion of Adolf Meyer ('95) and of 

 Ram.6n y Cajal ('04, p. 1051). The evidence for this conclusion 

 has been fully presented above. Here we point out merely: (1) 

 that the lower Amphibia, particularly in the early larval stages, 

 show a wide separation of the dorsal and ventral parts of the 

 median wall by a membranous septum such as to forbid the mi- 

 gration of cellular elements from the one to the other, except at 

 their rostral ends where they converge merely to receive their 

 olfactory tracts; (2) that the primordium hippocampi attains 

 its characteristic amphibian structure while this septum is still 

 membranous; (3) that the massive septum of Anura develops 

 wholly at the expense of the underlying nucleus medianus septi; 

 (4) that even here, where at no stage a membranous septum epen- 

 dymale is present, the septum and primordium hippocampi are 

 separated at all stages subsequent to the differentiation of their 



