90 Journal of Comparative Neurology. 



The cavity lies in the mesal portion of the cerebellum, the 

 part to become the vermis, and while of considerable extent in 

 the younger embryo, has in the older one become comparative- 

 ly much reduced by the juxtaposition of the opposite walls. 

 Later a fusion between the walls undoubtedly takes place, giv- 

 ing rise to the funnel-shaped depression as figured by Kohl- 

 mann. An attempt to homologize this cavity with that in some 

 teleosts and birds would be interesting. 



CHOROIDAL FOLD. 



During the middle of the second month in the human em- 

 bryo the pons flexure is instituted. It consists of a bending of 

 the whole nervous tube, so that the extremities of the rhom- 

 boidal fossa are approximated dorsally and finally the caudal 

 border of the cerebellum comes to lie in the vicinity of the cal- 

 amus region of the medulla. Figs. 6, ii, i8, 19 and 20. 



A fold in the roof is thus produced into which the mesen- 

 chyme insinuates itself and forms the plica choroidalis. Figs. 5, 

 6 and 11. At the same time the roof is carried laterally over 

 the sides of the medulla forming the lateral recesses. Figs. 13 

 and 20. (See page 100.) 



The villi are at first developed chiefly in the mesal portion 

 of the fold and later appear in the sides of the lateral recess. 

 At first in human embryos they are somewhat scattered over its 

 lateral wall, but are more discreet in embryos of other animals. 

 Compare Figs. 8 and 13. They soon coalesce to form a some- 

 what crescentic continuous body with its concavity cephalad, 

 reaching from the extremity of one recess to that of the other. 

 Later, two processes are developed, reaching caudad on either 

 side of the median line along the roof of the body of the ven- 

 tricle. Fig. 28. At the same time the choroid plexus, on ac- 

 count of the non-development of the mesal portion, appears to 

 become divided between the caudal extensions. The picture 

 now presented is that of a choroidal plexus starting in the ex- 

 tremity of each lateral recess, passing to the meson and then 

 extending caudad to the extremity of the ventricle. Fig. 16. 



The caudal portions form the paired choroidal plexuses of 



