loo Journal of Comparative Neurology. 



arrangement of the cells and by its taking certain stains more 

 readily. 



In the extension of the ventricle in the child at term and 

 the adult, which forms the walls of the metapore, the epithelium 

 gradually loses its character and can not be distinguished from 

 the arachnoidal endothelium. There is no abrubt termination. 



As to the time of the appearance of the metapore it is dif- 

 ficult to give exact figures. It was found by C. Hess in embryos 

 of 7, 12, 5, 1 6 and 17 cm., by Retzius in an embryo of 4 mos. 

 and by Gratiolet in one of 14 weeks. There is an element of 

 doubt in regard to these observations, especially in the younger 

 embryos, since the nature of the opening was not recognized. 

 I have found it in two embryos at the end of the fourth month, 

 the others so far studied being at too early stages. In embryos 

 of 22 and 42 mm. the epithelium of this region appears to be 

 very weakly developed and its continuity can only be deter- 

 mined with difficulty. 



LATERAL RECESSES. 



The lateral recesses are formed by the lateral portions of 

 the roof of the rhomboidal fossse, when the pons flexure is 

 completed. 



The fold in the roof formed by the flexure does not extend 

 across the roof at the region of its greatest breadth, i.e. oppo- 

 site the bend in the nervous parietes, but at a point caudad to 

 it which later corresponds to the caudal part of the entrance to 

 the lateral recess. Figs. 18 and 19. That is to say, the fold 

 in the roof is essentially not similar to a fold caused by the 

 bending of a flattened tube. Rather, the entire cephalic portion 

 of the roof is carried caudad over the oblongata. 



Thus it happens that there is no infolding of the roofs of 

 the lateral recesses since they are carried back with the cere- 

 bellar laminae. A reference to the figures will make the point 

 clear. The choroidal fold does not exist as such in the lateral 

 portions of the ventricle, the involution of the choroidal plex- 

 uses in the lateral recesses being a secondary formation and not 

 in a preformed fold. Figs. 4, 8, 9, 13 and 14. 



