98 Journal of Comparative Neurology. 



pia, and remarks on its resemblance to the condition found in 

 the horse. 



Judging from his description and figure, it is evidently an 

 instance of the condition found in the lower animals. He also 

 describes a thinning and perforation of the sides of the pouch 

 which throws light on the formation of the aperture. 



The hypothesis that there is an evagination of the roof 

 explains the excursion of the choroid plexus on the surface of 

 the worm. The roof of the pouch is here supported by its ad- 

 hesion to the pia of the cerebellum and consequently persists. 



The variations in length of the choroid plexus may depend 

 upon the extent of the evagination of the ventricle or upon 

 the development of the plexus itself. 



The supposition that the metapore might be the orifice of 

 an evagination has been advanced by Prof. C. S. Minot in a 

 communication to Prof. B. G. Wilder.^ 



In comparing the above described conditions as found in 

 the embryo with those found in the adult or child at term (for 

 there is no material difference between them) we find that the 

 difference exists merely in a more extensive obliteration of the 

 lateral walls and floor of the evagination, so that its characters 

 as a tube are lost. The numerous vessels in this vicinity encroach 

 upon the lumen of the tube, it becomes absorbed between 

 them, and thus arises the ragged appearance of this region. 



A suspicion that there is no break in the ventricular epithe- 

 lium and that the evagination becomes applied to the surface of 

 the arachnoid is refuted not only by the facts revealed by injec- 

 tion but by the mesh work of arachnoidal fibers that passes 

 between tonsillae, arachnoid, vermis and oblongata. 



In support of the injection method, I would state that the 

 epithelium has been found to persist only when supported by a 

 layer of tissue sufficiently strong to withstand considerable force. 



The comparatively few instances of complete closure of the 

 ventricle may be due primarily to a short or incomplete evagi- 

 nation, or to secondary adhesions between the arachnoidal or 



1 B. G. Wilder : The Metapore in Man and Orang. New York Med. News, 

 Oct. 14, 1893. 



