MORPHOLOGY OF THE FOREBRAIIM 465 



though I incline to believe that they are so, and therefore in rep- 

 tiles I retain the name fissura arcuata, for I think it marks the 

 position of the embryonic fissura arcuata and also that of the 

 mammalian fissura hippocampi. Elliot Smith ('03, p. 469) in 

 discussing the fissura arcuata of foetal monotremes, calls atten- 

 tion to a slight sulcus, /3, between the fascia dentata and the para- 

 terminal body (see fig. 71, where I present a copy of his figure of 

 foetal Echidna), which he calls the sulcus limitans hippocampi, 

 and he adds, "most writers on the reptilian brain regard the sul- 

 cus j8 as the homologue of the Bogenfurche (5) ; this drawing shows 

 how erroneous such a contention is." As a matter of fact, "most 

 writers on the reptilian brain" have not distinguished at all be- 

 tween the two fissures marked 8 (Bogenfurche or fissura arcuata) 

 and (sulcus limitans hippocampi) and my purpose in calling 

 attention to the matter here is to do so. In adult reptiles the 

 two fissures are sometimes distinct and the lips of the fissura 

 arcuata are bounded above by cortex hippocampi and below by the 

 undifferentiated primordium hippocampi. In other cases one or 

 the other may be wanting. 



Of these the fissura limitans is phylogenetically the older, for 

 it is homologous with the fissure which I have so named in the 

 Amphibia, while the true fissura arcuata is not found below the 

 reptiles. (The so-called fissura arcuata of the frog is the fissura 

 limitans, for it lies entirely ventral to the primordium hippocampi, 

 while the fissura arcuata always lies within the hippocampal forma- 

 tion.) It is true, as implied by Elliot Smith's remark quoted 

 above, that the fissura arcuata of the adult reptile and the sulcus 

 limitans of the foetal Echidna both mark the ventral limit of 

 the differentiated cortex of the hippocampal formation ; but they 

 are not homologous, for they do not occupy the same position 

 with reference to the other structures of the hemisphere. The 

 cortex hippocampi and fascia dentata nave in Echidna grown 

 downward through the primordium hippocampi into contact 

 with the precommissural body on the median surface of the hemis- 

 phere. The result is that, whilst in the adult reptile differentiated 

 cortex is found in a position which corresponds with the dorsal 

 lip only of the foetal fissura arcuata, in the mammal such cortex 



