UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 39 



The process of regression of the area membranacea superior in the human 

 embryo differs somewhat from that described in the pig. This alteration in the 

 mode of disappearance is largely due to the fact that in the period of growth 

 from 20 to 35 mm. the superior portion of the roof of the fourth ventricle in the 

 human embryo is not sacrificed to the cerebellar lips; for in the human the cere- 

 bellum grows largely into the fourth ventricle, enlarging beneath the superior part 

 of its roof. Thus, the attachment of this part of the roof is not greatly interfered 

 with by the rapid development of the cerebellum. The total extent, then, of the 

 superior portion of the roof is hardly altered in these stages in the human, while 

 in the pig embryo the roof is shortened by its attachment to the inferior portion 

 of the cerebellar lip, which retains its earlier characters. These differences in the 

 relationship of the superior portion of the ventricular roof in human and pig embryos 

 may be seen by comparison of figures 74 and 89. 



Another factor which renders the mode of disappearance different in the two 

 embryos concerns the greater tufting and development of the chorioid plexuses of 

 the fourth ventricle in the pig. This greater size and complexity of the plexus 

 causes an encroachment upon the roof structures which, in the pig embryo, seems 

 of considerable importance in the final closure. 



In the human embryo, however, it has been found very difficult to explain the 

 final disappearance of the superior membranous area on the same mechanical 

 factors which seemed so well to account for its transitory characters in the pig; but 

 at approximately the same stage of growth the process of regression occurs in the 

 human fetus. The area maintains a fair size in stages up to a length of 23 mm. 

 Thus, in figures 89 and 90 (No. 453 of the Carnegie collection) a sagittal section 

 from a human fetus of this size is illustrated. In the higher power (fig. 90) the 

 superior membranous area is shown, rather sharply delimited on its superior border 

 by the typical, dense ventricular ependyma. Below, its edge is irregularly formed 

 by the deeply staining ependyma over the imagination of the chorioid plexus. The 

 cell-character of this area resembles that shown in the photomicrographs from the 

 specimen of 21 mm. (figs. 64 and 65). There is left in the area no indication of the 

 cellular architecture which characterized the original ventricular ependyma; the 

 cells with their elongated cytoplasmic processes here have the oval nuclei which are 

 found almost invariably in this membranous area. 



In the human fetus of 26 mm. (No. 1008 of the collection of the Carnegie 

 Institution of Washington) there is but slight evidence of a superior membranous 

 area in the upper portion of the roof of the fourth ventricle. The evidence present 

 in this specimen consists in a localized thickening of the lining cells of the ventricle 

 in the situation of the area in other stages. This thickening is illustrated in figures 

 91 and 92; it consists of several layers of epithelial-like cells, similar in all respects 

 to the many-layered border shown in figure 83. The picture is somewhat obscured 

 by the vascular plexus directly beneath the ventricular lining. 



There is difficulty in determining exactly when the last evidences of the supe- 

 rior membranous area in the roof of the fourth ventricle may be found. This is 



