UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 43 



holds. The measurements given above were taken from mounted sections and are 

 probably somewhat disturbed by the histological technique which was followed. 



The borders of this oval area membranacea are usually fairly regular and 

 smooth, but in some instances they are irregular, due to the fact that small exten- 

 sions of the area run into the bordering ependyma. These extensions are more 

 commonly met with at the stage when the area has reached its maximum size, 

 as in figures 38 and 39, photomicrographs from an embryo pig of 19 mm. The 

 higher power of these two photographs shows two areas in the smoother ependymal 

 wall. These are extensions of the area membranacea, and within a section or two 

 directly connect with the differentiated area. Both of these small spots on the 

 circumference resemble technical errors; their ragged appearance, the relative exca- 

 vation of their surface, and the intact ependymal borders would seem to encourage 

 such a view; but when considered in connection with the character of the whole 

 area membranacea they assume a definite relationship in this regard. Other similar 

 areas, rather rare in occurrence, are found separated entirely from the main area 

 membranacea. These isolated areas are of the same size as those shown in figure 39. 

 In significance and character they are probably identical with the larger area mem- 

 branacea superior. 



Most of the general features of the area membranacea superior have been com- 

 mented upon in descriptions of the various stages of differentiation in both pig and 

 human embryos. The characteristics most commonly observed concern the dif- 

 ferentiated character of the cells of the area, the sharp borders of the typical epen- 

 dyma, the ragged ventricular surface throughout the whole extent, and the peculiar 

 adhesion of the albuminous coagulum from the embryonic cerebro-spinal fluid 1 to the 

 lining cells. The area membranacea superior should be considered, then, as a transi- 

 tory focus of differentiation of the typical ependymal lining of the roof of the fourth 

 ventricle. 



AN UNDESCRIBED AREA IN THE INFERIOR PORTION OF THE ROOF OF THE FOURTH 



VENTRICLE. 



With success attending the effort to find in the superior portion of the rhombic 

 roof an anatomically differentiated area which would furnish a morphological basis 

 for the physiological phenomenon of the extraventricular passage of the cerebro- 

 spinal fluid, attention was necessarily directed to the inferior portion of this roof 

 (considering the whole roof structure to be divided by the chorioid plexuses). The 

 spread of the replaced injection fluid (fig. 4) into the periaxial tissues through two 

 points in the roof of the ventricle suggested a study of this stage (pig embryo of 

 18 mm.) as the basis of the investigation. As a histological ly differentiated area in 

 this inferior portion of the roof is easily made out, the complete history of the area 

 will be given chronologically. It has been termed the "area membranacea inferior 

 ventriculi quarti," the terminology being based on the same physiological and 

 anatomical features which led to its adoption in the case of the analogous area in the 

 upper portion of the roof. 



