86 Journal of Comparative Neurology. 



tinction between the two is not marked and is rather one of 

 degree than of absolute limitation. 



In my descriptions I designate the portion giving direct or- 

 igin to the roof plate as the secondary lip and reserve the pri- 

 mary lip for the bending of the substance of the dorsal lamina. 



The directions of the two lips are different as a rule, but 

 in the region of the lateral recess they coincide and it is im- 

 possible to make a definite separation of them in later stages. 

 Figs. 4, 8, 30, 34, 36, and 38. 



In the cerebellum the differentiation of the lips is very 

 evident, the primary consisting of a large portion of the cere- 

 bellar plate (dorsal lamina) so that when the folding is com- 

 pleted the attachment of the secondary lip is carried to the 

 vicinity of the isthmus. Figs. 6, 7 and 8. 



The primary rhomboidal lips fuse in the calamus region 

 and produce a bridge of nervous matter over the emergence of 

 the myelocoele into the fourth ventricle. This appears as a well 

 marked projection in sagittal sections at the meson. Figs. 24, 

 25 and 26. 



The secondary lips in the same region have a caudal bend 

 over the closed portion of the oblongata and may or may not 

 fuse across the median line. When they do they form the 

 obex. Figs. 16, 17, 24 and 26. The obex appears to be in- 

 constant not only in man but in the lower animals as well. In 

 three adult rabbit brains it appeared once. The obex is entirely 

 intraventricular as is evinced by the attachment of the roof 

 epithelium to its caudal margin. Figs. 24 and 26. In frontal 

 sections advancing cephalad of the obex the secondary lip appears 

 as a well marked rather vascular body which can be readily recog- 

 ized by the naked eye as staining deeply, especially in advanced 

 embryos. Figs. 28 and 31, human; Fig. 33, cat. It is usually 

 closely fused with the oblongata, but the line of division as a 

 rule can be identified by the entrance of vessels. 



In the human foetus at term, Figs. 31 and 32, it bends 

 sharply over into the roof of the ventricle, giving rise to two 

 nervous laminae which in this region form the major portion of 

 the roof. When the roof is torn away, the margin where the 



