492 PEOF. G. ELLIOT SMITH OX THE MORPHOLOGY OF THE 



structure. With regard to that part of tlie plexus which is found in the region of the 

 foramen of Monro, there can be little doubt of its origin from the primitive roof of the 

 forebrain. Its developmental history clearly shows that this part of the choroid plexus 

 was never anything else than a simple epithelial layer, and a comparative study of its 

 behaviour in different Vertebrates indicates that it ought to be regarded as a derivative of 

 the primitive roof, or " Beckplatte" of the forebrain. To speak of it as a portion of the 

 cerebral hemisphere which has undergone a retrograde change, or has retained its 

 primitive epithelial simplicity, is to make a gratuitous assumption, which receives no 

 confirmation from the facts of embryology. But the case is very different with that 

 portion of the choroid plexus which is not directly connected with the roof of the fore- 

 brain, but is attached to the stria terminalis. There is no evidence to show that this 

 portion is derived from the roof, and all the facts of development point to the conclusion 

 that its proximal attachment to the optic thalamus is a primitive and not a secondarily 

 acquired relation. Such being the case, the caudal extension of the epithelial choroidal 

 fold in the Mammalian hemisphere would appear to be derived from a stretching of the 

 attachment of the labium caiidale of the cerebral hemisphere to the optic thalamus. As 

 a result of this the connecting band becomes reduced to an epithelial lamina, which 

 becomes invaginated and folded by an extension backward of the choroidal folding which 

 begins farther forward in the region of the foramen of Monro. This hypothetical 

 explanation of the derivation of the posterior choroidal fold is grapliically demonstrated 

 in the accompanying schemata (figs. 27 & 28, p. 491). 



No full history of the mode of development of the lateral choroid plexus has ever been 

 given, so far as I am aware. Most recent writers admit the undoubted origin of its 

 cephalic part from the roof, and either ignore the more caudal portion or frankly admit 

 their ignorance of its morphogenesis. The hypothesis which I have suggested to 

 explain the possible mode of production of its caudal portion receives some su.pport from 

 comparative studies, and is advanced on the present occasion because the examination of 

 the plexus in Sphenodon seems to have an important bearing upon the question at issue. 



In Sphenodon the two attenuated portions of the mesial wall of the hemisphere — 

 using this expression in its ixsual but inexact sense — are separated the one from the 

 other by the commissiira aberrans. At the situation of the latter the ventral margin of 

 the paraterminal body becomes linked to the corresponding part of the other hemisphere, 

 and thus a bridge is formed which effectvially limits the choroidal fold. Behind the 

 situation of this commissure the process of attenuation of the connecting band between 

 the hemisphere and thalamus may be readily recognized. It is not difficult to conceive 

 how, in a brain such as that of a Mammal in which there is no barrier in the shape of a 

 conimissitra aberrans, the epithelial invagination which begins in the region of the 

 "porta" (foramen of Monro) may extend in the caudal direction and involve the thin 

 portion of the mesial wall of the hemisphere, which has a different mode of origin. This 

 hypothesis might explain the jieculiar attachments of the post-portal part of the choroid 

 plexus. 



The existence in the brain of Spheuodon of a thin mesial hemisjihere-wall, which has 

 luidoubtedly become attenuated secondarily and forms a part of the true wall of the 

 hemisphere-vesicle linking the caudal part of the hijjpocampus to the optic thalamus, 



