18 WILLI AM F. AIJ.KN' 



completely obliterate that j^ortion of the embryonic central 

 canal. iVside from the change in the central canal the size of the 

 medulla has increased in all directions, but especially laterally, 

 due doubtless to an increase in the number of nerve fibers and 

 cells in the lateral plates. Both roof and floor plates are very 

 thin, comprised of about one layer of nuclei each. The ventral 

 plate may be slightly thicker, due to the addition of a few nerve 

 fibers to the outer layer. Absolutely no stretching of the roof 

 plate has occurred, indicating that it is not under any marked 

 internal pressure. In figure 37 (B.V.) branches of the inter- 

 segmental blood vessels have stretched out toward the roof of 

 the brain, but at this stage they are too remote from the roof 

 plate to be very active in infiltration. Figure 39 makes clear 

 that the spinal cord has made equal progress with the medulla 

 in developing a cleft-like or typical embryonic central canal. 

 In the cord the dorsal and ventral enlargements of the central 

 canal are shown to be of about equal size. 



An additional day (figs. 40 and 41) discloses considerable 

 expansion of the central canal throughout, with the exception 

 of the extreme ventral portion. Very noticeable is the increase 

 in size of the dorsal portion, the future fourth ventricle (cavita 

 della tela coroidea of Sterzi). The roof plate exhibits no signs 

 of stretching. Up to this stage the expansion appears to be due 

 to the migration of the ependymal cells upward and outward, 

 rather than to pressure within. The floor plate will be seen to 

 have increased considerably in thickness through an addition 

 of fibers to its marginal layer, which would obviously tend to 

 make the floor plate more resistant than the roof plate to pressure 

 from within from this time on. Both lateral plates disclose a 

 remarkable growth in thickness due to an addition of both fibers 

 and cells, some of which, however, must be attributed to the 

 fact that the head was sectioned somewhat obliquely' (note in 

 fig. 40 that the V and VIII ganglia appear in the same section). 

 But little progress has occurred in the development of the inter- 

 segmental blood vessels, so that Sterzi's conjecture, that the 

 embryonic cerebro-spinal fluid does not differ in any way from the 

 ordinar}^ intercellular fluids, would probably hold, if it could be 



