537 



the choroid plexus as being wholly a derivative of the roof until I 

 examined the developmental history of this part of the brain in Sphen- 

 odon. lu this reptile the choroid plexus is certainly formed partly 

 from the roof of the forebrain , but partly also from a secondarily 

 thinned part of the wall of the hemisphere (op. cit., Trans. Linn, Soc, 

 p. 492). In the neighbourhood of the foramen of Monro (Fig. 18) the 

 epithelial roof of the forebrain is pushed into the lateral ventricle {PL). 

 But further back the thalamo-pallial junction becomes stretched and 

 attenuated (just as in Lepidosiren, Fig. 4, y) to form the caudal part 

 of the lamina chorioidea. The invagination of the roof membrane near 

 the foramen of Monro gives rise to the "choroidal fissure", which in 

 many vertebrates (though not in Sphenodon) becomes prolonged back- 

 ward into that part of the lamina chorioidea, which is the result of 

 the attenuation of the attachment of the pallium to the optic thalamus. 

 The comparison of the morphogenesis of the brains of Lepidosiren, 

 Sphenodon and Ornithorhynchus shows that in the Dipnoan and Mammal 

 (though not in the Reptile) a third element contributes, though in quite 

 ,a small degree, to the formation of the lamina chorioidea. This third 

 element is produced by the attenuation of that part of the paraterminal 

 body (Fig. 18, P), which lies on the caudal side of the lamina ter- 

 minalis. 



The consideration of these facts enables us to realize the nature 

 of the unique modification of the vertebrate brain, which is found in 

 Ceratodus. Burt G. Wilder^) describes the choroid plexus in Cera- 

 todus as an invagination into the ventricle of a part of the wall of 

 the hemisphere, which is devoid of true nervous parenchyma; and goes 

 on to state that its relations are like those of other vertebrates, but 

 with the diti'ereuce that the choroidal fissure of Ceratodus proceeds 

 forwards from the foramen of Monro, whereas that of all other 

 Vertebrates passes backwards from the corresponding place. Un- 

 fortunately the attachments of this plexus are not clearly shown in 

 Bing's illustration (op. cit., Taf. 42, Fig. 2) of a section through the 

 brain of Ceratodus and his description does not give the necessary 

 information, so that there is some element of uncertainty as to the 

 exact condition of affairs. But it is possible that what happens in 

 Ceratodus is as follows. 



Starting from the condition shown in stage 48 (Bing and Burck- 

 HARDT, op. cit., Textfig. 15, p. 550), in which the hippocampus extends 



1) The Dipnoan Brain— Ceratodus. American Naturalist, Vol. 21, 

 1887, p. 544 (quoted from Bing and Burckhardt, op. cit. supra, p. 515). 



