80 . WILLIAM F. ALLEN 



The so-called second roof plate expansion appears in model 

 2 (figs. 4 and 5, R.Ex. 2) immediately behind the first. This is 

 a much smaller outcropping of the roof plate. It contains a 

 cavity (figs. 16 and 17, C.C.Ex.), which spreads out a little lat- 

 erally and caudally, and communicates below with the central 

 canal. This cavity is filled with a fine fibrillar feltwork that 

 stains deeply with orange G, and which for the most part is 

 coagulated cerebro-spinal fluid. The ependeymal walls of both 

 the roof expansion and the central canal are sufficiently vascular 

 to suggest that we have here as in the previous roof expansion, 

 a modified chorioid plexus which is producing cerebro-spinal 

 fluid. 



In model 2 (figs. 4 and 5, R.Ex. 3) the third outcropping of the 

 roof plate is some little distance behind the second, about equal- 

 ing it in size. This roof expansion has not been figured in trans- 

 verse section, but from an examination of a graphic reconstruc- 

 tion of the central canal, three small isolated cavities were seen 

 extending in a cephalo-caudal direction. The middle cavity was 

 found to be in communication below with the central canal. It 

 held cerebro-spinal fluid, and its walls apparently functioned in 

 the production of the same. 



It is evident that these three expansions of the roof plate are 

 independent of one another. The arrangement of the three 

 small isolated cavities in the third expansion seems to be an 

 embryonic condition, and suggests that the larger cavities may 

 have been produced by the union of several smaller roof expan- 

 sions. It may be supposed that these expansions were formed 

 by the multiplication of roof plate cells, which were pushed up 

 in solid masses, in which vacuolization and confluence of adja- 

 cent cavities produced the larger cavities seen in the adult. 



It is of interest to note that in this individual a posterior sinus 

 (fig. 20, S.T.), probably representing the sinus terminalis of a 

 normal individual, was isolated by the complete occlusion of the 

 central canal by ependymal tissue. The ependyma surrounding 

 this cavity is very vascular, and the cavity is distended with 

 cerebro-spinal fluid. This sinus is much larger than the fourth 

 ventricle in a normal individual. 



