92 Journal of Comparative Neurology. 



mammals. In man, with the exception of the choroid plexus, 

 it consists largely of two distinct medullary laminae which spring 

 from the ligula on either side. Figs. 31 and 32. These nervous 

 plates extend caudad beyond their direct attachment to the lip 

 over the apex of the ventricle and they are here attached laterally 

 by means of the pia, lined only by a thin epithelial layer. Fig. 

 31. In five brains of children at term examined especially with 

 this in view essentially the same condition appeared in all. The 

 epithelium continues on from the caudal margin of these plates 

 over the pia of the vicinity for a short distance and then grad- 

 ually fades away. 



This formation of the roof seems to be peculiar to man. 

 In no other animal did the roof appear otherwise than as thin 

 membranous tela consisting of a layer of epithelium lying on 

 the pia. 



The reason for this appears in the greater development of 

 the secondary rhomboidal lip in man. 



This characteristic appearance of the roof is apparent in hu- 

 man embryos at the beginning of the third month. Compare 

 Figs. 2 and 3 1 . 



As will be seen, the differences in the extent of nervous 

 matter are even more pronounced in the lateral recesses. 



Although the roof is better developed in man, it seems to 

 be more persistent in the lower animals, and we have to turn 

 to them to find an elucidation of the condition which appears 

 in the caudal portion of the roof in man. 



In advanced embryos of dogs, cats, pigs, sheep and also 

 the chick, a marked caudal protrusion of the ventricle appears, 

 which consists of the ventricular epithelium surrounded by a spec- 

 ial condensation of the mesenchymatic tissues. This protrusion 

 is completely closed and resembles the finger of a glove. Figs. 

 21, 22 and 23. The floor of this pocket springs directly from 

 the caudal margin of the secondary rhomboidal lip, which is 

 here folded over backward, while the walls are directly continu- 

 ous with those of the ventricle. Its lumen at its commence- 

 ment corresponds to the width of the ventricle and is circular in 

 section. It terminates abruptly in a rounded extremity. 



