538 



as far as the attachment of an epithelial membrane, far more attenu- 

 ated than that seen in Lepidosiren in Figure 16, the thinning of the 

 dorsal part of the paraterminal body extends forward (compare Figs. 15 

 and 16, P) for a considerable distance (even perhaps the greater part 

 of the length of the hemisphere): and when the invagination of the 

 roof membrane begins (at the foramen of Monro) to form the choroid 

 plexus (compare Fig. 17 PL.\ the process of folding extends forward 

 in the attenuated portion of the paraterminal body. 



This may also afiford the clue to the transformation of the form 

 of the brain in the Teleostean and Ganoid fishes. While I agree with 

 Studnicka in regarding the so-called "pallium" of the bony fishes as 

 mainly a stretched roof -membrane, analogous to the tela chorioidea, 

 and look upon the so-called basal ganglion as including the homologue 

 of the true pallium (as well as the real basal ganglia), it seems im- 

 possible to explain the foreward extension of the thin membrane (so- 

 called "pallium") into the olfactory peduncle, unless we admit that 

 this part of the wall of the hemisphere has undergone a process of 

 secondary thinning, analogous to, though far more extensive than, 

 that, which occurs in Ceratodus. 



There is only one other point that I shall refer to at present. The 

 very pronounced clumping of the cells at the two margins — mesial 

 and lateral — of the pallium in Lepidosiren (Fig. 1) is an excellent 

 illustration of a similar tendency which is of widespread occurrence in 

 the Amniota. It is no doubt due to the fact that at the two situations, 

 where the olfactory tracts — mesial and lateral respectively — enter 

 the pallium, a closer grouping of cells (and ultimately a specialization 

 of structure) occurs to form receptive organs for the incoming olfac- 

 tory impressions. The lateral clump is the first indication of the 

 origin of the pyriform lobe and the mesial clump of the fascia den- 

 tata. Giuseppe Levi^) has given an admirable account of the onto- 

 geny and fate of the mesial clump of cells in the Mammalia, and has, 

 moreover, added his testimony to mine that such an aggregation of" 

 cells occurs at the mesial edge of the pallium in certain Reptiles 2). 

 It is only in a few reptiles that a difference in texture can be de- 

 monstrated between the ventral (mesial) edge and the rest of the 

 medio-d(Msal cortex. 



1) Ueber die Entwickelung und Histogenese der Ammonshorn- 

 formation. Arch. f. mikrosk. Anat. u. Entwickelungsgesch., Bd. 64, 1904, 

 p. 389. 



2) Sull'origine filogenetica della formazione ammonica. Archivio 

 di Anat. e di Embriol., Vol. 3, 1904, Ease. 1, p. 284. 



