( Coniihucd fraiii ikkji ').', ) 



L't) imd L'7 licpicsciil I wo ( rniisvcrsc seel ions tliroiiuli I lie nir(|iill;i, piissiiiK 

 tliroUKli til'' |-.isl(M-i(.r cii.l of llic luiirlli vculiiclr ..f llic s;iiiic scries iron: \vlii<-li 

 tiglllTs -Jl and L':> were drawn. Xolc csi.ccially liic cxiiansion of the rool' plalc 

 and coniparc willi the so-called second roof phitc; expansion of tlic 20 cm. Polis- 

 totrcnia spinal cord i li)j;. Itii. ll is (pii'st ional)le vvhothor tlio opcninsR (C) arc: 

 artifacts or not. As was noted previously, the inesencliynie outside the roof 

 phite is very vascular and tiie loof pla I e has t he apjx'arnce of heino; under anioder- 

 ate (letiree of intci-nal pi-essure. :< 10. 



L'S .V ratheidl.li(|ue rr(udal section tliroujih t he medulla of a i:. mm. human 

 end)ryo (Inst, of .\nat.. Iians, seiies, II L'o), In (his region the contour of the 

 rhomhic Kr.ain is such that t he i)osterior part of the roof expansion of the fourtli 

 ventricle is cut t ransversely w Idle t he more antericji- portion, seen below, appears 

 more or less in fiontal section. More anterior sections would show the roof 

 plate to 1)1' continuous. The [josterior end of the fourth ventricle will admit of 

 direct comparison with the second roof expansion of the 20 cm. Polistotrema 

 spinal cord (fig. 10). Had the fourth ventricle been empty, as was the case of 

 the rubber tubing in His' experiments, there would be absolutely no grounds 

 for believing that the anterior portion of the roof plate would be expanded as 

 it is by the appearance of a pontine flexure. It might on the contrary have been 

 folded up within the ventricle. X 16.6. 



29 A transverse section through the extreme posterior end of the fourth 

 ventricle of the same series as figure 28. There is here a slight roof plate expansion 

 containing no cavity. Compare with figure 15. X 16.6. 



30 Transverse section through the thoracic spinal cord, taken from the same 

 series as figure 28. Note that the roof plate consists of ependyma only, while 

 the floor is reinforced by white matter, and even at this late stage if any marked 

 increase in pressure occurred from the cerebro-spinal fluid of this region, an 

 expansion of the roof plate would have been entirely possible. X 16.6. 



31 Similar to figure 28, but from an 8 mm. human embryo ("Inst, of Anat., 

 series H4). In this plane the posterior end of the fourth ventricle is cut nearly 

 transversely, and is directly comparable with the second roof expansion of the 

 20 cm. Polistotrema spinal cord (fig. 16). The cavity was full of coagulum, its 

 walls have the appearance of being under moderate internal pressure, and the 

 adjacent mesenchyme is very vascular. X 46.6. 



ABBREVIATIONS . 



B.V., blood vessel 01., inferior olive 



C, an apparent communication be- R.Ex., roof plate expansion 



tween the veno-lymphatics and the R.L., rhombic lip 



fourth ventricle R.P., roof plate of the central nervous 



C.C., central canal system 



Ch n., chondrocranium S.C., semicircular canals 



C.P., choroid plexus of the fourth S.R., sensory or dorsal spinal nerve 



ventricle root 



Crh.L., lateral lol)es of the cerebellum T.S., tractus solitarius 



C.T., white fibrous connective tissue VIII. G., auditory ganglion 



E])., ependyma V.L.S., veno-lymphatic sinus 



Ex.Ar., external arcuate fibers V.R., trigeminal root 



/.L., inner or ependj'mallayer of nuclei W.R.F., white reticular formation 



Mar.L., marginal layer XI. R., accessory nerve root 

 M.L., mantle layer 



56 



