Blake, Roof of the Fourth Ventttcle. 99 



pial surfaces, the epithelium being absorbed. The evidence in 

 favor of an incomplete evagination as a factor in absence of the 

 metapore is, that in these cases the choroid plexus does not 

 extend back on the vermis. 



In any case we must look upon an extension of the chor- 

 oid plexus into the subarachnoid cavity as evidence as to the 

 presence of the metapore. 



The metapore is essentially not an aperture in a roof but 

 rather the disappearance of the end of a structure, so that it 

 can be compared to the opening of a shed. Hence many of 

 the current descriptions are incorrect. 



Its boundaries are not definable in all cases since its posi- 

 tion depends upon the amount of absorption of the epithelial 

 tube. Its apparent boundaries are those given by Key and Ret- 

 zius, namely the choroid plexus above, the membranous epi- 

 thelial walls on either side, and below the posterior margin of 

 the obex if present, otherwise the ligula. The propinquity of 

 vessels and arachnoidal strands may give it the ragged appear- 

 ance described by some. 



Its resemblance to a tube has been recognized by Retzius 

 and emphasized by Hoffman and Rauber and by C. A. Morton. 



Of great interest and value as bearing upon the nature of 

 the metapore are the appearances of the ventricular epithelium, 

 especially in the roof and its extensions. 



These changes are best followed in embryos, for in them 

 they have not proceeded so far as to entirely obscure its 

 characters. 



The epithelium lining the floor of the ventricle is colum- 

 nar and ciliated, on the secondary lip it becomes lower as it ap- 

 proaches the margin and loses its ciliae. Where the roof is sim- 

 ply epithelial it is of a low cuboidal character. 



In older embroys these changes become more accentuated. 

 Where the roof is membranous the epithelium becomes flat- 

 tened and the cells much broader, so that they approach closely 

 the appearance of the endothelial cells of the arachnoid. In the 

 evagination of the roof in the lower animals this appearance is 

 marked but the epithelium can be recognized by a slightly closer 



