UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 47 



THE AREA MEMBRANACEA INFERIOR IN THE HUMAN EMBRYO. 



The same process in the formation of an area of differentiation in the inferior 

 portion of the roof of the fourth ventricle may also be followed in the human 

 embryo. Unfortunately, however, human embryological material can rarely be 

 subjected to the immediate fixation and preservation which yield excellent histo- 

 logical results in the more plentiful specimens. It does not seem strange, therefore, 

 that the determination of the exact stage at which an area of differentiation can be 

 made out in the ventricular roof should be practically impossible; for, in poor tech- 

 nical procedures, the roof of the fourth ventricle suffers almost more than does any 

 other portion of the specimen. 



In a human embryo of 13 mm. (No. 695 in the collection of the Carnegie Insti- 

 tution of Washington) there is slight evidence of a differentiation in the lower por- 

 tion of the rhombic roof. The changing character of cells in this specimen is not 

 marked, but as the central portion of this inferior roof is reached the ependymal 

 cells seem to assume gradually a more cubical morphology. Associated with this 

 change in shape, there is also a slight loss of the deeply staining character of their 

 nuclei. The whole differentiation, however, is slight and would be commented upon 

 only from the conception of this area in the pig embryo. 



The first definite evidence of differentiation in the inferior portion of the ven- 

 tricular roof was found (specimen 390 in the Carnegie collection) in a human 

 embryo of 15.5 mm. This initial differentiation occurs, then, in the human embryo 

 of approximately the same length as in the pig. The specimen showed the same 

 change in character of the lining ependyma as was found in the pig. The deeply 

 staining ependymal elements are replaced in a limited central area in the inferior 

 portion of the roof by cells with more elongated nuclei, poorer in chromatin, and 

 resembling somewhat the epithelial-like cells which early filled the ventricular roof. 

 These cells tend to compose a layer of more than one cell in thickness a feature 

 particularly noticeable in the peripheral portions. 



The size of the area membranacea inferior observed in specimen 390 suggested 

 that the earliest evidence was probably to be observed in somewhat smaller speci- 

 mens. This could not, with the material at my disposal, be verified, but it is prob- 

 ably safe to assume that the first signs of an ependymal differentiation will be found 

 in human embryos of about 15 mm. This time of appearance of the area in the 

 human would coincide with its time of primary differentiation in the pig embrj^o. 

 In this limitation of the first appearance of the area membranacea inferior, the 

 standard has been an unmistakable differentiation of ependyma and not an isolated 

 change of a lining-cell or two which might have been the result of the technical 

 procedure. Such a criterion was necessitated by the very marked changes in the 

 ventricular borders observed in specimens in which distortion of the chorioidal roof 

 had occurred. 



The area membranacea inferior very rapidly increases in extent after the onset 

 of the process of ependymal differentiation. This was likewise observed in the pig 

 embryo, although perhaps more stages could be made out. In a human embryo of 



