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



THE AREA MEMBRANACEA SUPERIOR IN THE HUMAN EMBRYO. 



The finding of the differentiated area in the superior portion of the roof of the 

 fourth ventricle in the embryo pig suggested the value of a study of the same region 

 in the human embryo in the further solution of the problems underlying its occur- 

 rence. Hence this region in the roof of the fourth ventricle has been examined in 

 the sectioned human embryos of the Department of Embryology of the Carnegie 

 Institution of Washington. It was found that a similar area occurred hi the human 

 embryo of approximately the same age. 



The study of the roof of the fourth ventricle is usually more difficult in the 

 human embryo than in the pig. This is due to the fact that the roof of the fourth 

 ventricle quickly suffers from poor fixation and dehydration collapse or inversion of 

 the whole structure being commonly met with. It is rarely possible, in the younger 

 embryos, to secure the most satisfactory fixation, whereas in the pig these factors 

 may be controlled as desired. Furthermore, the undue pressures to which the 

 human ovum is frequently subjected in abortion may cause crushing of the more 

 delicate parts of the nervous system. 



It is probably best, in the human embryo as in the pig, to trace the formation of 

 the area membranacea superior ventriculi quarti from its beginning, through the 

 various differentiations. 



In a human embryo of 4 mm. (No. 836 of the Collection of the Carnegie Insti- 

 tution of Washington) the entire roof of the fourth ventricle is composed of cells with 

 round or slightly oval nuclei and palely staining cytoplasm. The nuclei of the cells 

 are poor in chromatin material as contrasted with the pyknotic character of the 

 typical ependymal cells. The lining tissue is of the thickness of several cells. The 

 ventricular cytoplasmic border is fairly smooth at this stage. This characteristic 

 ventricular lining is shown in figures 40 and 41, both taken from embryo No. 836. 

 The whole picture is similar to that exhibited by the pig embryo of 8 mm. (figs. 24 

 and 25). 



A similar accumulation of epithelial-like cells is found in a human embryo of 

 7 mm. (No. 617 of the Carnegie collection). This is pictured in figures 48 and 49. 

 The photomicrograph of higher magnification shows these poorly staining cells 

 heaped up in a rather localized part of the ventricle, fairly sharply delimited from 

 the adjoining ventricular lining. This accumulation of cells in the roof of the ven- 

 tricle invariably occurs, and it must not be considered as being due to the distortion 

 of the ventricular roof. The reason for the asymmetry of the rhombic roof shown 

 in these figures lies in the fact that in this embryo, as in practically all the embryos 

 of similar stages in this collection, some degree of distortion of the roof of the fourth 

 ventricle is present. Photomicrographs (figs. 50 and 51) taken more posteriorly 

 (from embryo No. 617) give strong evidence of this distortion. They are repro- 

 duced not only to show the possible distortion, but also to give a further picture 

 of the lining of the ventricle, with its epithelial-like cells in several layers (fig. 51). 



Similar accumulations of these epithelial-like cells are to be found in human 

 embryos of 9 mm. Reproductions of a much fragmented specimen of this size 



