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



16 mm. (No. 406 of the collection of the Carnegie Institution) the area membranacea 

 inferior is quite extensive, as is shown in figures 80 and 81. In the photomicro- 

 graph under higher power (fig. 81) the densely stained ependyma approaches the 

 membranous area (ami) as tongue-like processes from above and below. These tips 

 gradually lose their dense character and are prolonged as a delicate membrane, 

 lining, in this localized area, the ventricular cavity. The nuclei of the cells here are 

 not heavily laden with chromatin; they are oval and somewhat larger than the 

 more densely packed nuclei of the typical ependymal element. Unfortunately, the 

 middle portions of the membranous area in this specimen are surrounded by extrav- 

 asated red blood-cells obscuring somewhat the structure (fig. 81). The process, 

 though, of the differentiation of these ependymal elements into paler and larger 

 epithelial-like cells is quite apparent. 



As in the pig, the tendency of the differentiated ependymal cells forming the 

 area membranacea inferior to lose in some degree their distinctive appearance and 

 to approach in character the undifferentiated mesenchymal element is apparent in 

 the human embryo very shortly after the original steps in the process of differen- 

 tiation have occurred. Photomicrographs from two human embryos of 17 mm. have 

 been included to show this phenomenon. Thus, in figure 88, an enlargement of the 

 blocked area from figure 58, the area membranacea inferior (ami) is well defined. 

 The sagittal section from which this photomicrograph was taken is from embryo 

 No. 576, in the Carnegie collection. Above and below the dense line of ependyma 

 may be made out; this tapers quite abruptly, to be succeeded by the cells of the area 

 membranacea inferior. These cells, products of ependymal differentiation, have 

 lost much of their epithelial-like appearance; they now show rather small, oval or 

 rounded nuclei, poor in chromatin. The cytoplasm of the cells is small in amount, 

 but not disproportionate for the size of the nucleus. The ventricular border of 

 these cells (fig. 88) exhibits a rather characteristic phenomenon, the adherence of a 

 slight albuminous coagulum. The fine processes of this coagulum fuse with the cyto- 

 plasmic borders of the cells and render these borders vague and indefinite. Beneath 

 the cells of this inferior area small vascular channels may be made out. These tend 

 to make the membrane appear denser than its cellular character warrants. 



In another section from this same embryo (No. 576) the inferior membranous 

 area is shown in relation to the tufted chorioid plexuses (figs. 82 and 83). In the 

 reproduction under higher magnification (fig. 83) the ependymal lining may be traced 

 caudalwards to a gradual fusion into the area membranacea inferior. From the 

 rather high cubical cells in the immediate proximity to the plexuses the ependymal 

 elements become reduced in size and in height, and then rather abruptly the pyknotic 

 character of the ventricular lining is lost. This loss of the deeply staining character 

 coincides with the superior border of the area membranacea inferior (ami). The 

 membrane of this area shows the same cell-character as already described for this 

 embryo. On the superior side of the plexuses (fig. 83) the lateral border of the area 

 membranacea superior (ams) is shown composed of epithelial-like cells. 



