80 PEK(;iVAL BAILEY 



UISTOHV 



Concerning the structures with which this study is concerned, 

 the writings of such authors as Faivre ('54), Luschka ('85), 

 and Haeckel ('60), previous to the work of Wilhehn His, contain 

 very Httle of vakie. 



Of the choroid plexus of the lateral ventricle, His writes ('04) : 



Soin deni Thalamus angehefteter Randstreifen bleibt ependymal und 

 in ihm bildet sich die Fissura chorioidea, von der aus die Epithelfaltun- 

 gen des Corpus chorioideum in den Seitenvontrikel sich einstiilpen. 



Minot ('01) considers the lateral plexus to be developed from 

 the velum transversum. 



In both birds and mammals the lateral portions of the velum, i.e., 

 the choroid plexus of the lateral ventricle is highly developed. It 

 thus appears that as we ascend the vertebrate series there is first a 

 broadening of the velum, and an increase in its lateral development, 

 then occurs a further reduction and flattcnino- out of the velum, and a 

 much greater growth of the lateral plexus. 



G. Elliot Smith ('03) attempted another explanation for the 

 formation of the lateral plexus. 



Now, although in the whole of its extent the epithelial layer of the 

 choroid plexus presents uniform features, it is difficult to admit a com- 

 mon origin for the whole structure; with regard to that part of the 

 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 fore- 

 brain But the case is very different with that portion of the 



plexus which is not directly connected with the roof of theforebrain, 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 thala- 

 mus is a primitive and not a secondarily acquired relation. Such lieing 

 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 caudale 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 i-egion of the foramen of Monro. 



Johnston ('09) after showing that the velum transversum is 

 continued down the side-wall of the prosencephalon as the di- 

 telencephahc groove, returns to a modification of Minot's 

 original idea. 



