MORPHOLOGY OF THE FOREBRAIN 449 



COMPARISONS WITH FISHES 



The brief account of the brain of Lepidosiren recently published 

 by Elliot Smith ('08) shows striking resemblances with the Am- 

 phibia, with, however, divergent lines of specialization. The 

 differentiation of a layer of cortical cells in the dorsal region is 

 reptilian in type, though these cells are probably represented in 

 the frog also (see fig. 40), and the ventral enlargement of the tuber- 

 culum olfactorium is unique. The similarity to the amphibian 

 conditions is, in fact, so close as to justify us in reviewing Elliot 

 Smith's conclusions in the light of the preceding discussion. 



He describes the corpus paraterminale as comprising practically 

 the whole median wall dorsally of the tuberculum olfactorium 

 and further states ('08, p. 533) that it is divided into dorsal and 

 ventral parts by an " indentation of epithelial cells" which the 

 figures show to mark the level of the interventricular foramen. 

 The ventral part is clearly the nucleus medianus septi. It is 

 equally clear that the dorsal part is chiefly composed of the equi- 

 valent of the amphibian pars dorso-medialis, or primordium hip- 

 pocampi. In the adult frog Elliot Smith has recognized ('03, p. 

 497) the distinction between the primordium hippocampi and that 

 portion of the paraterminal body which lies dorsally of the inter- 

 ventricular foramen (pars fimbrialis septi of Kappers) ; but he 

 has failed to make the corresponding analysis here. An exam- 

 ination of the photographs reproduced in his paper suggests that 

 the resemblance to the Amphibia is even closer than Elliot Smith 

 seems to have recognized. 



The arcuate fissure is absent in Lepidosiren, as in the Amphibia. 

 The fissure B of Elliot Smith's fig. 1 would appear at first to be the 

 fissura limitans hippocampi, which marks the position of the zona 

 limitans in the frog; but this I think is not the case. It is more 

 nearly comparable with the ependymal groove which is seen in 

 the frog larva (fig. 30), and marks the level of the dorsal boundary 

 of the interventricular foramen. In both Lepidosiren and the 

 frog tadpole the precommissural body (nucleus medianus septi) 

 extends dorsally of this groove, while the fissura limitans hippo- 

 campi always lies wholly dorsal to the precommissural body. The 



