UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 37 



(No. 721) are given in figures 52 and 53. In the latter figure the complete occupa- 

 tion of the ventricular roof by these cells is well illustrated. Moreover, the speci- 

 men shows the many-layered stage to a degree but seldom found. It is unfortunate 

 that such a degree of fragmentation and distortion is found throughout this specimen. 



Thus far, in human embryos up to and including 9 mm. in length, the roof 

 of the fourth ventricle has shown the same architecture as appears in the pig. As 

 will be recalled, the first evidence of a further differentiation of these cells in the pig 

 embryo was found at a stage of 11 mm. (figs. 26 and 27). In one human embryo 

 of this stage (No. 544) a distinct break in the roof of the fourth ventricle can be 

 made out. This is shown in two photomicrographs (figs. 54 and 55). The picture 

 in this case is somewhat obscured by the shrinkage and distortion of the ventricular 

 roof, but a distinct differentiation of the lining epithelium can be made out. On the 

 caudal side of figure 55 considerable nervous tissue is seen. Just superior to this 

 (toward the left) the lining tissue is almost lacking, a few nuclei, only, preserving 

 the contour of the ventricle. Above this area appears again the ventricular lining 

 of many layers of cells. It has been quite difficult to interpret these findings. The 

 area under discussion shows a rather typical adherence to the coagulated albumen; 

 there is evidence of its extension also into the adjacent mesenchyme, a finding 

 observed in no other similar stage. The caudal position of the opening, the char- 

 acter of the tissue approximating the ventricular cavity, and the presence of the 

 albumen in large amount in the adjacent mesenchyme all indicate that in great 

 measure the pictures presented in this specimen are largely artifacts. It seems 

 most likely, though, that some differentiation of the tissue in this area has occurred. 



In a human embryo of 14 mm.,* as in the pig of the same stage, the area 

 membranacea superior has attained a great degree of differentiation. This is 

 particularly well shown in figures 56 and 57, the latter being an enlargement of the 

 squared area in the former. These photomicrographs are from embryo No. 144 of 

 the collection of the Carnegie Institution of Washington. Figure 57 shows a charac- 

 teristic which distinguishes the area membranacea from that of the pig, although in 

 the later stages of the pig embryo (figs. 45 and 47) this feature is present. This 

 concerns the marked decrease of cellular tissue in the membranous area. In figure 

 57 the deeply staining typical ependyma is shown approaching from below. These 

 cells end abruptly at the border of the area membranacea; the ventricle in this area 

 is lined by cells possessing small elongated nuclei and long cytoplasmic processes, 

 which unite to form a ventricular lining. The oval nuclei along the ventricular 

 border become more closely massed together in the superior portion of the area, but 

 nowhere is there the same architecture as hi the equivalent stage in the pig (fig. 33) . 

 A feature of the histological appearance of the membranous area in the pig embryo 

 is also shown in figure 57; this is the marked adherence of the coagulated albumen 

 of the cerebro-spinal fluid to the area membranacea superior. 



The roof of the fourth ventricle in the human embryo is subjected to the same 

 factors causing changes in the form and relationships which were commented upon 



*Measured on the slide after mounting. 



