UNDESCRIBED STRUCTURES IN ROOF OP THE FOURTH VENTRICLE. 41 



more posteriorly than the two preceding figures. The dense ependyma approaching 

 on both sides is sharply delimited at the edge of the broad membranous area. This 

 is composed of cells having elongated, chromatin-poor nuclei, and long cytoplas- 

 mic processes, which form the ventricular roof. The adherence of the albuminous 

 coagulum occurs here also. 



In the rabbit the occurrence of the superior membranous area was verified as 

 in the other species studied. In a rabbit embryo of 13 mm. (series x in the embryo- 

 logical collection of this laboratory) the area was well differentiated from the sur- 

 rounding typical ependyma. The cells of the area resembled those of the adjacent 

 mesenchyme. The ventricular surface was roughened by the projection of num- 

 erous protoplasmic processes. An albuminous coagulum was attached to the cells 

 of the membranous zone. 



One sheep embryo from the collection of this laboratory was also studied. The 

 sections, although labeled as an embryo of 10 mm., resembled in every wa.y a pig 

 embryo of 18 mm. The area membranacea was easily identified in the roof of the 

 fourth ventricle ; it is similar in every respect to the same area in the pig and the 

 human embryo. 



In a cat embryo of 10 mm. a small but highly differentiated area membranacea 

 superior was made out. The most striking feature in this specimen is the great 

 adherence of the coagulated albumen to the cells of the area and the resemblance of 

 these cells to the mesenchymal elements adjacent. The edges of this differentiated 

 area are sharp and clear-cut. 



No attempt was made to identify the area membranacea superior in other 

 animals as further suitable material was not immediately available. The chief 

 study has been made on pig embryos and on human embryos. The occurrence of 

 the area in the cat, sheep, and rabbit probably indicates its existence in all mammals. 

 The finding of such an area in the chick is also suggestive. 



GENERAL CONSIDERATION OF THE AREA MEMBRANACEA SUPERIOR. 



The occurrence of a definite area of differentiation in the superior portion of the 

 roof of the fourth ventricle has been pointed out in preceding subdivisions of this 

 paper. It has been described in detail in the pig embryo and in the human embryo; 

 it has been identified also in cat, sheep, rabbit, and chick embryos. It remains here 

 to discuss the general characteristics of this area. 



No description of such an area of differentiation in the ventricular roof has 

 been found in the literature. It may be that the distortion of this structure in 

 the course of the usual embryological technique has rendered its discovery less 

 likely. His' 25) , in his description of the ventricular roof, has not commented upon 

 the occurrence of this membranous area, even though in a retouched photomicro- 

 graph of his fetus C-l (a human specimen, of the beginning of the third month) the 

 area membranacea superior can be made out. Likewise in his description of the 

 plica chorioidea he fails to mention any differentiated areas in the roof, although 

 plate i, in his " Die Entwickelung des menschlichen Rautenhirns, von Ende des ersten 



