UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 49 



The apparent tendency of the cells composing the inferior membranous area 

 to lose the epithelial-like character, as shown in the figures from embryo No. 576, is 

 not an invariable phenomenon. Rather is an aggregation of epithelial-like cells met 

 with in human embryos very commonly in this area, not only in embryos of small 

 size, but also in small fetuses. This phenomenon is illustrated in figures 84 and 85, 

 reproductions of photomicrographs from a human embryo of 18 mm. (No. 409 in the 

 collection of the Carnegie Institution). In figure 85 the total transverse extent of 

 the area membranacea inferior (ami) is illustrated, with the villous chorioid plexuses 

 appearing to the left. Although this membranous portion of the embryo has been 

 distorted somewhat by the technical procedures to which the specimen was sub- 

 jected, the cellular character of the membranous area is well indicated. The most 

 striking feature, apart from the characteristic tinctorial differentiation from the 

 typical ependymal elements, consists in the marked clumping of the cells in certain 

 parts of the membrane. On one lateral extent the membrane is thickened into a 

 bulbous swelling several cells in thickness. These cells have palely staining nuclei, 

 poor in chromatin, with an oval or round form. In other places in the membrane 

 smaller but no less characteristic clumps of similar cells may be made out. Between 

 these cellular aggregations the membrane stretches in a continuous line with but few 

 nuclei. 



Analogous clumps of cells, with pale, rounded or oval nuclei, may be made out 

 in figures 86 and 87, taken from a human embryo of 19 mm., No. 431 in the collec- 

 tion of the Carnegie Institution. Only a small portion of the membrane is repro- 

 duced in the figure under higher magnification, but a characteristic clump of epithe- 

 lial-like cells (epc) is shown. These cells of the differentiated ependyma here again 

 have oval and rounded nuclei, poor in chromatin, similar to those which have been 

 pointed out many times in the foregoing pages. A second broadened area in the 

 inferior membrane is also shown in figure 87. 



The further development of the area membranacea inferior proceeds in the 

 human embryo in a manner very similar to that described for the pig. In the stages 

 but slightly above those already described the differentiation goes on slowly, but 

 within a few millimeters the cellular pictures resemble those given for the embryo of 

 17 mm. (figs. 82, 83, and 88). The cellular clumps which appeared quite frequently 

 in the embryos under 20 mm. have not been found in the larger forms. Thus, in an 

 embryo of 23 mm. (No. 453 in the collection of the Carnegie Institution) the inferior 

 membranous area (ami) appears as an extensive membrane comprising almost 

 wholly the inferior portion of the chorioidal roof. The membrane is here of a single 

 cell in thickness; these cells are rather small, with oval nuclei, simulating in some 

 measure those of the surrounding mesenchyme. The most interesting phase of the 

 membranous area at this stage of 23 mm. concerns its completed cellular differen- 

 tiation and its rather slow increase in size. 



Wholly similar pictures of the inferior membranous area of the roof of the 

 fourth ventricle are afforded by a human fetus of 26 mm. (figs. 91 and 92). These 

 photomicrographs were taken from embryo No. 1008 in the collection of the Car- 



