40 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



due to the likelihood of artifacts disturbing the character of the ventricular lining 

 in human material, where the freshness and fixation of the specimen may not be 

 ideal. In the larger specimens in the collection of the Carnegie Institution, which 

 are well fixed and sectioned, the existence of the area membranacea superior could 

 not be wholly verified. Thus, in specimen 405 (26 mm.) the presence of the area 

 seemed probable though not definite. In another embryo of this same size (No. 782) 

 the existence of this area was still more questionable. In a larger embryo (30 mm., 

 No. 75) the presence or absence of the area could not be assured; many indications 

 suggested its existence, but the resemblance to an artificially separated ependyma 

 was strong. In all specimens of human embryos of over 30 mm. examined, no evi- 

 dence of the area membranacea superior could be found. It appears likely, then, 

 that the final disappearance of this differentiated area in the roof of the fourth ven- 

 tricle occurs at a slightly earlier stage in the human embryo than in the pig. 



The final disappearance of the area membranacea superior in the human embryo 

 is not accompanied by the same ingrowth of typical ependyma that characterizes 

 the process in the pig. There is a great tendency, in the human, as indicated in 

 figure 92, for a replacement of the area by the same type of epithelial-like cell which 

 comprised the whole ventricular roof in the earlier stages (fig. 41) and later formed 

 lateral borders for the superior membranous area (fig. 83). Thus, in a human 

 embryo of 24 mm. (No. 632 of the Carnegie collection) there is evidence of a very 

 small membranous area surrounded by a border of epithelial-like cells. In a slightly 

 larger specimen (No. 840, 24.8 mm.) the whole membranous area is occupied by the 

 epithelial-like cells. The frequent association of these cells with the area indicates 

 that in disappearing the area membranacea is probably replaced first by these cells, 

 which in turn disappear, so that the whole roof is finally composed of the typical, 

 densely staining ependyma. 



THE AREA MEMBRANACEA SUPERIOR IN OTHER ANIMALS. 



In order to ascertain whether the area membranacea superior existed in other 

 animals examinations of serial sections of the rabbit, cat, sheep, and chick of suitable 

 stages were made. All of these animals were found to possess a differentiated area 

 in the roof of the fourth ventricle. 



Opportunity was afforded for the study of serial sections of the head of a chick* 

 of 121 hours' incubation. The head was carefully dehydrated and embedded by 

 Dr. E. R. Clark, and was subsequently sectioned by Dr. C. R. Essick. The material 

 was beautifully fixed and dehydrated, showing practically no evidence of shrinkage. 

 Typical portions of the superior membranous area are reproduced in figures 66, 67, 

 68, and 69. Figure 67, taken near the crown of the embryo and representing the 

 squared area in figure 66, shows the two dense masses of ependyma separated by the 

 more lightly staining area membranacea. The cellular character of this differen- 

 tiated zone resembles more thehistological features of the similar area in the pig than 

 those of the human embryo. This resemblance is also to be seen in figure 69, taken 



*This chick measured 14 mm. in 40 per cent alcohol. 



