535 



type we are accustomed to regard as distinctive of the Amniota. 

 The condition of affairs represented in Lepidosiren (Fig. 17) is 

 exactly analogous to that found in foetal mammals, more especially 

 those (the Monotremata and Marsupialia) , which have retained the 

 dorsal part of their hippocampal arc, undisturbed by a corpus callosum. 

 In the brains of foetal Ornithorhynchus and Perameles, which I have 

 represented 1) , the same arrangement of thalamencephalon , lamina 

 chorioidea and mesial edge of the pallium (i. e. hippocampus) is found. 

 A similar relationship will be found in the homologous parts of the 

 brain in certain Keptiha, such, for example, as many Chelonia. But 

 in certain other reptiles (such as Sphenodon, the Lacertilia and some 

 of the Ophidia) there is a very striking difference in the arrangement 

 of the nervous tissues fringing the cerebral mantle, which is, at first 

 sight, extremely puzzling, and has given rise to great confusion in the 

 literature relating to the comparison of the reptilian and mammalian 

 forebrain. I have described this region of the brain in a foetal Sphen- 

 odon (Hatteria) in considerable detail 2), and v. Kupffer ^) , in his 

 admirable account of the morphogenesis of the brain, has represented 

 the condition of affairs in Anguis fragilis, and given an interpretation 

 of them (p. 238), to which I shall have occasion 

 to refer. 



If a coronal section be made through the 

 cerebrum of a foetal Sphenodon or Anguis so 

 as to pass through the foramen of Monro 

 (Fig. 18), and this be compared with a cor- 

 responding section in Lepidosiren (Fig. 17), it 

 will be found that whereas, in the Dipnoi (as 

 also in the Chelonia and Mammalia) the lamina 

 chorioidea is attached laterally to the edge of 

 the pallial formation, i. e. the hippocampus, 

 in these particular reptiles a caudal prolon- 

 gation of the paraterminal body (Fig. 18, P) 

 is interposed between the attachment of the 

 lamina chorioidea and the pallial formation. 

 Most recent writers (see, for example, v. Kupffer, 

 op. cit, p. 238) have not unnaturally assumed 



Thalam- 

 encephalon 



Formatio 

 pallialis 



Fig. 18. A diagram 

 representing the relation- 

 ships of the structures found 

 in the neighbourhood of 

 the foramen of Monro in 

 a foetal Sphenodon (Howes' 

 stage E). To be compared 

 with Fig. 17. 



1) Op. cit. supra, Quart. Journ. Microsc. Sc, 1896, PI. 2, Figs. 15 

 and 16. In both of these drawings I have erroneously applied the 

 name "paraphysis" to the dorso-cephalic corner of the thalamencephalon. 



2) Op. cit. supra, Trans. Linn. Soc, especially Fig. 9, p. 462. 



3) Op. cit., Hertwig's Handbuch, Pigs. 256 and 257, p. 236. 



