UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 35 



usually developed to such an extent that a continuation of the former size becomes 

 impossible. This is shown in figures 44 and 45. Figure 45, the enlarged squared 

 area from figure 44, is a photomicrograph from a pig embryo of 23 mm. The dif- 

 ferentiated area, due to the factors favoring its regression, now appears in close 

 proximity to the chorioid plexus. It has more the appearance of a degenerating 

 area at this stage than in any of the younger embryos, but it still shows a character- 

 istic delimitation of both edges on the one from the typical ventricular ependyma, 

 and on the other from the differentiated ependyma of the chorioid plexus. The 

 cytoplasmic strands of the area which forms the ventricular border do not show to 

 advantage in the photomicrograph, but the same ragged character with the covering 

 of coagulum may be made out. The process of regression, mechanical as it perhaps 

 is, has begun at this stage in the pig, and in the course of the next few millimeters' 

 growth will become even more active. 



With the encroachment of the chorioid plexuses and the downward growth of 

 the cerebellar lip, the superior portion of the ventricular roof soon disappears, and is 

 practically non-existent in embryos of 30 mm. and more in length. The differ- 

 entiated area thus encroached upon from the sides and above becomes a mere 

 vestige of its former size. Thus in a pig embryo of 32 mm. (figs. 46 and 47) it 

 appears as a very small break in the lining continuity of the ventricular ependyma. 

 Without the intermediate stages such a picture would undoubtedly be considered 

 as an artificial erosion of the ependymal lining of the ventricle, but when studied 

 in connection with figure 45 the true vestigial character of the area becomes estab- 

 lished. 



The final fate of this differentiated area in the roof of the fourth ventricle is a 

 complete disappearance, with the occupation of the region by chorioidal epithelium 

 and cerebellum. In this study it was impossible to find traces of the differentiated 

 areas in pig embryos of over 33 mm. in length; vestiges may persist, but so small as 

 to present difficulties of decision. The persistence of such a differentiated vestige 

 in rare instances would not be surprising; the transitory character of the area and 

 the method of disappearance make this seem not unlikely. 



This transitory area of differentiation in the roof of the fourth ventricle of the 

 pig has not, so far as can be determined, been noted or described by any previous 

 author. His (25) , in a retouched photomicrograph of a sagittal section of a human 

 embryo of 17 mm., reproduced the area as differentiated from the roof, but he has 

 made no comment upon it. I have called this differentiated area in the superior 

 portion of the rhombic roof ventricle the "area membranacea superior veutriculi 

 quarti." This terminology is based on the anatomical character of the area as a 

 continuous membrane, but chiefly on its physiological significance. For, as will 

 be shown in the succeeding section of this paper, the transit of embryonic cerebro- 

 spinal fluid from ventricle to periaxial tissue occurs in this area, which functions 

 apparently as a physiological membrane. With such a physiological conception 

 of the area, the term "area membranacea" seems most suitable, inasmuch as it 

 also meets the anatomical requirements. 



