Blake, Roof of the Fourth Veiitncle. 85 



so far as to form a lip but gives rise to what simulates the 

 Flugelwialste of His, i. e., the rounding formed by the growth 

 of the rhomboidal lip. 



Its existence in rabbit embryos has been denied by Dexter. 



Lately^ I have been able to demonstrate conclusively its 

 existence in the rat embryo of three millimeters N. B. length. 

 Fig. I. 



Its development does not proceed so far in the lower mam- 

 mals as in man, especially in the region of the cerebellum. It 

 certainly does not achieve the morphological importance attrib- 

 uted to the rhomboidal lip by His in man, namely, the cover- 

 ering in of the funiculus solitarius and the formation of the raphe 

 and olive, unless we can consider this process to take place by 

 an emigration of the neuroblasts from the lip rather than by 

 an actual folding of the lip around the medulla, which latter 

 seems to me an unsubstantiable proposition. 



The existence of a primary rhomboidal lip in a non-fused 

 condition in the lower animals, is probably extremely transit- 

 ory, which explains its hitherto apparent absence. With the 

 formation and fusion of the rhomboidal lip the secondary 

 rhomboidal lip makes its appearance. 



The secondary lip is essentially a transition between the 

 nervous parietes and the roof and it partakes in its constitution 

 of the characters of both. It commences as a lamina of nervous 

 matter and thins away more or less quickly into the roof. 



In man it reaches its greatest development, but it was 

 present in all the embryos and adult animals studied. 



The primary rhomboidal lip is narrow in the calamus re- 

 gion and then broadens somewhat in the region of greatest 

 width, but reaches its greatest extent in the cerebellar region. 

 Figs. 5, 6, and 8. 



His states that in the calamus region it later becomes sep- 

 arated from the sides of the oblongata , but I am inclined to 

 think that what appears as a separation is really the develop- 

 ment of the secondary lip. In any case in this region the dis- 



1 Within a few weeks after the reading of this paper. 



