Blake, Roof of the Fourth Ventncle. 93 



The choroid plexus extends in its roof a short distance 

 caudad of the extremity of the vermis. 



In the dog at birth the vermis has not as yet covered in the 

 ventricle, which is still visible a short distance behind it. Tran- 

 sections through this region appear exactly similar to those of 

 the same region in man. There is the same extension of the 

 choroid plexus along the under surface of the vermis. If the 

 S^tions are followed back serially, or if a sagittal section is 

 examined, it will be evident that an evagination of the ventricle 

 has been cut, giving rise to the appearance of a break in the 

 roof. Fig. 25. This evagination differs slightly in its relations 

 at this stage from the condition found in the earlier ones, in 

 that the roof of the evagination has become fused to the opposed 

 surface of the vermis. 



The floor of the evagination still remains separate from 

 the pia of the oblongata. An explanation of this can be found 

 in the relations of the mesenchymatic tissues which form the pia 

 and arachnoid. In all young embryos this forms a delicate but 

 close meshed network between the brain and the parietes of the 

 cranium. As growth proceeds rarefactions and condensations 

 of this network appear which arrange themselves according to 

 fixed rules, and thus the subarachnoid spaces and the mem- 

 branes are formed. Fig. 2 1 . 



It is out of the province of this paper to treat further of 

 this subject except to state that in the region under considera- 

 tion the network between the vermis and the evagination of the 

 ventricle becomes condensed, while the other portions about 

 the evagination become rarefied into the postcisterna. The con- 

 densed portion dorsal to the evagination forms quite a strong 

 close meshed layer, which has been recognized and described by 

 Hess, who attributed to it the drawing up of the choroid plexus 

 on the vermis. In the horse he states that it is especially well 

 developed, and below is attached to a process of pia shaped 

 like the finger of a glove, which closes in the ventricle. He also 

 implies that it draws out this pocket-shaped process. His im- 

 pressions are seen to be erroneous from both points of view, 

 inasmuch as the pocket is preformed, and secondly, does not 



