528 



Ramon y Cajal^), however, has adopted another way of looking 

 at the matter and has given his reasons for it. He believes that the 

 problem of the homology of the mesial wall of the hemisphere in 

 Keptilia "has not been finally solved" and that "the field is still free 

 for other conjectures". He suggests that this part of the reptilian 

 hemisphere may correspond to that (neopallial) area of the mesial 

 wall in the mouse and Guinea-pig, which presents many structural 

 analogies. I have discussed the reasons for not adopting such a view 

 at great lengtli in my memoir of 1902 (op. cit. supra), with which 

 Ramon y Cajal was not acquainted. But, briefly, the reasons for 

 identifying this area with the hippocampus are as follows. 1) In the 

 two most primitive and diversely-specialised mammalian orders — the 

 Monotremata and Marsupialia — the hippocampus is not confined to 

 the postero- ventral edge of the pallium, but fringes its whole extent, 

 as far forward as the neighbourhood of the olfactory peduncle; 2) in 

 all other mammals definite traces of this extensive hippocampal arc 

 can be found fringing the whole extent of the mesial edge of the 

 pallium; 3) therefore, presumably, the ancestors of the Mammalia must 

 have had a hippocampus in the corresponding position ; and 4), in the 

 Reptilia we find a state of afi'airs which is a faithful and exact copy of all 

 the essential features of this part of the brain in the foetal Monotreme 

 or Marsupial (see especially Figs. 12—19 in my memoir, op. cit. supra). 

 No hypothesis as to the homology of the mesial pallium of the 

 reptile, which does not account for this cephalic part of the hippo- 

 campal arc, can be seriously considered. 



I must take it for granted that the fact of the identity of the 

 hippocampus in the reptile is definitely established. 



If we return to the consideration of the section represented in 

 Figures 1 and 2 and study its details in the light of the data brought 

 together in my memoir (op. cit. supra, p. 499), there can be no hesi- 

 tation in recognising the formatio pallialis as a true pallium, the mesial 

 part of which is certainly homologous with the hippocampus of the 

 Reptilia and Mammalia. Nor can there be any real doubt that the 

 part of the mesial wall of the hemisphere which immediately adjoins 

 the hippocampus is the paraterniinal body. But how much of the 

 mesial wall is to be considered as forming part of that body is a 

 matter calling for discussion. The outer edge of the formatio pallialis 

 is a thick mass of cells at the point marked A. 



1) Studien über die Hirnrinde des Menschen, 1906, Heft 5. (Brbs- 

 leb's Translation.) 



