{Cnilnnn.lfrnm i>a,,, .5S) 



:>(; toiiS Ivoiu a I 1 .lay I'd roiii.v/.on crnhiyo IIhoiikIi Mic saiiic icnic.iis as I lio.so 

 shown in liLimfs 'M\ lo .'!,"), and to racililale coinparisun were placed dircu^tly un- 

 der lliein. Consideralde proiiress iuis occurred everywhere. Note Ij that 

 I he (•(•ntrai canal scam with its small dorsal :ind ventral cavities in the 10 nun. 

 sci'ics has ^^ivcn place to a 1\ pical cmUryonic central canal, whicii is much wider 

 al the top and hottom than at the center. The constricted jiortion of coui'se 

 rei)resents the last place for tlu; protoplasm to give way or to be disintegrated. 

 ■J) 'i1ie floor |)late is slijilitl>- thicker and less expanded than the roof plate, being 

 riMnl'orced on the outside by white matter and by a rapidly growing notochord 

 lielow .')) A r(>w more nerve fibers and nuclei have appeared in the lateral 

 plates. I) 'I'he number of dividing germinal cells has increased while the num- 

 ber of yolk graiudes remains about the same. 5) The first blood vessels have put 

 in appearance directly outside the meningeal membrane (fig. 37, B.V. and 

 nearer the roof plate on the opposite side). X 125. 



.39 Transverse section through the spinal cord of the same 11 day Petromyzon 

 series as figures 36 to 38, showing the so-called typical embryonic si)inal 

 cord. X 125. 



40 and 41 Somewhat obliciue transverse sections from a 12 day Petromyzon, 

 passing in figure 40, through the V ganglion on one side and the VIII ganglion on 

 the opi>osite side, and in figure 41 through the X ganglion on one side and a 

 region l)ehind the X ganglion on the opposite side. They can readily be compared 

 with the 11 day series above (figs. 36-38). The lateral plates have apparently 

 increased notably in the number of nerve fibers and nuclei, some of which, however, 

 will have to be attributed to the fact that the sections are cut quite obliquely. 

 Also the numbers of nerve fibers have increased in the floor plate. Throughout, 

 the central canal has increased in width. Of especial interest is a small central 

 mass of protoplasm (Sy.P.) in figure 40, which for a space of 50 microns per- 

 sists as the last remnant of a once solid mass of protoplasm in the center of the 

 medulla. It is obvious at this stage that some factor must have produced suf- 

 ficient internal pressure to i)revent the closing up of the ventricle on account 

 of the rapid increase of cells and fibers in the lateral plates. It is fair to assume 

 that this factor is internal pressure from cerebro-spinal fluid. X 125. 



54 of this plate will be described in its proper place, opposite the next plate. 



60 



