SPINAL CORD AXD MEDULLA OF CYCLOSTOMES 29 



such a connection is wanting (fig, 13). The fact that in various 

 places (fig. 13) there is more or less of a string of ependymal 

 cells between the walls of the central canal and the roof plate 

 expansion suggests that in a more embryonic state an open com- 

 munication existed between the roof plate cavity and the central 

 canal, which became closed in the region of the posterior end of 

 the roof expansion, and persisted in the anterior end. Another 

 variation to be noted in the cavity of the posterior end of the 

 roof plate expansion is that it contains numerous islands and 

 promontories of ependymal cells and connective tissue, shown 

 as white spaces in figure 6. These probably represent portions 

 of the roof plate that have not been completely excavated to 

 form a continuous cavity. 



Transverse sections 11 to 13 show the cavity of tliis first roof 

 plate expansion to be larger than the fourth ventricle in Polisto- 

 trema (figs. 64 to 66), and the whole structure more nearly 

 resembles a typical fourth ventricle than does the fourth ven- 

 tricle itself in this animal. For the most part the walls of the 

 roof plate consist of true ependymal cells, ditTering in no way 

 from those surrounding the central canal, except for their shorter 

 peripheral processes. Posteriorly connective tissue takes the 

 place of many of these cells. One of these cells is shown in 

 figure 13A to be sufficiently granular to suggest a secretory func- 

 tion. Everywhere the walls of both the roof plate and the cen- 

 tral canal are very vascular, suggesting a modified choroid 

 plexus. Figure 13A will demonstrate the ease witli which inlil- 

 tration and diffusion could take place between the blood vessels 

 of the roof plate and its cavity. In figure 11a fold of the roof 

 expansion, containing a blood vessel, will be seen extending 

 into the cavity, and about it there is collected a mass of coagu- 

 lated cerebro-spinal fluid {S.C.F.). 



Of still greater interest are the two posterioi- outcroppings of 

 the roof plates, designated as roof plate expansions 2 and 3 

 (figs. 4 and 5, R.Ex. 2 and 3), Since these two roof plate expan- 

 sions are considerably smaller than the first, they can be com- 

 pared directly with the roof plate of the rhombencephalon of 

 anv embrvo. 



