CEREBEAL COMMISSURES IX THE ^TIRTEBRATA. 469 



to determine to a larg:e extent the peculiar shape of the hippocampal formation and the 

 presence of the hippocampal fissure. 



If it were pertinent to the subject of this communication, I might introduce here a large mass of data 

 in support of these statements regarding the morphogenesis of the hippocampal formation [vide "The 

 Fascia dentata," Anatomischer Anzeiger, 189(J], and in support of Hochstetter's contention that the 

 hippocampal fissure is a much later feature to make its a})pearance than the supposed " Bogenfurche/' 

 and that the latter has a purely post-mortem existence. 1 have, howevci, referred to this question only 

 to point out that the fascia dcntata makes its appeatance comparatively late in the development of the 

 brain *, and is derived from the hippocampal formation by a specialization of its ventral edge. 



Before leaving this section I wish to call attention to a furrow {(3) which separates the 

 paraterminal body (a) from the hippocampal formation, and which I may call " sulcus 

 limitans [hipj^ocamjn]" Most writers on the Reptilian brain regard the sulcus fB as the 

 homologue of the Bogenfurche (8) : this drawing (fig. 16) shows how erroneous such a 

 contention is. 



Fig. 17. 



pdra.. 

 Scheme of a section through the rccessus superior of a foetal Ormthorhijnclius. 



A section thi'ough the recessiis superior of a foetal Oj'nitJiorhijuchus (fig. 17), at a much 

 earlier stage than the Uchidna just considered, shows the "neopallium" (ne.) joining the 

 hippocampus (hip.) at the dorso-mesial angle of the hemisphere just as it does in Echkhia ; 

 but there is no definite fascia dentata, although a slightly exuberant mass of cells [f-d.) 

 represents the earliest jjhase of its development. [The other features of this section do 

 not concern us here and have been described elsewhere. " The Brain of a FcEtal 

 Oniit/ior/ii/iicf/us," Quart. Journ. Micr. Sci. vol. xxxix. p. 181.] 



In the light of the facts wliich this brief examination of the relations of the java- 

 terminal body and hippocampus has revealed, it is interesting to examine the condition 

 of the analogous parts of the brain in Reptiles. For this purpose I have deliberately 

 chosen the brain of a Saurian, not because I suspect any close phyletic relationship 

 between the Saurians and the Monotreinata, but because the arrangement of the homo- 

 logous regions in the two types of the brain is so clearly identical, that the comparison 

 between the two becomes an extremely simple matter. The question whether this 



* The foetus under consideration had in other respects attained to almost the adult type, and yet the fascia 

 deutata is in the primitive condition shown in the figure. 



