UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 31 



AN UNDESCRIBED AREA IN THE SUPERIOR PORTION OF THE ROOF OF THE 



FOURTH VENTRICLE. 



THE AREA MEMBRANACEA SUPERIOR IN THE PIG EMBRYO. 



Examination of the roof of the fourth ventricle in a pig embryo of 14 mm. 

 revealed a peculiarly differentiated area in the superior portion. The general topog- 

 raphy of this area is shown in the rectangular area marked off in figure 32 a median 

 sagittal section from a pig embryo of this critical stage. In figure 33 this rectangular 

 area is enlarged to show the morphology in greater detail. 



In this figure the densely staining ependyma lining the fourth ventricle ap- 

 proaches from both sides. The superior portion of the ependyma ends abruptly, 

 while the inferior line of the layer tapers more slowly. Between these two points is 

 an area having none of the characteristics of the ventricular lining at all other points. 

 The comparatively smooth contour of the ependymal cells is replaced by an irregular 

 cell-border. The pyknotic nuclei of the cells have been replaced by less densely 

 staining, elongated, spindle-like nuclear bodies. The cell-layer lining the ventricle is 

 here really only of a single cell in thickness, although blood-capillaries closely applied 

 to it suggest a greater thickness. The mesenchyme between this layer and the 

 peripheral epidermis is quite thin, but resembles in every way the mesenchyme in 

 the immediate neighborhood. 



There is, therefore, as pictured in figures 32 and 33, an area in the roof of the 

 fourth ventricle which is morphologically dissimilar to the characteristic ependyma 

 lining the cavity. Is this the result of some distortion in fixation or in the routine 

 histological technique? Is it a constant finding and, if so, what is its history? Does 

 it arise at a definite period and persist throughout intra-uterine life only or through 

 adult life also? 



The question of the actual existence of this area, or of its being caused by 

 technical manipulations, is one which must be answered. That this differentiated 

 portion of the roof of the fourth ventricle is not an artifact is verified by the general 

 history of its formation, by its invariable occurrence (not only in the pig but in 

 other animals), and by its general histological appearance. Moreover, the physio- 

 logical importance of this area undoubtedly inclines one completely from the 

 possible explanation that it is due to an artifact. No single finding wholly excludes 

 such a possibility; rather is one convinced, by many features, of its actual occurrence. 



Considering the fact, then, that this differentiated structure in the roof of the 

 fourth ventricle may be found in all embryo pigs at the stage of 14 mm., it becomes 

 necessary to ascertain at what time in the development of the embryo it first appears 

 and how it is formed. Obviously the most satisfactory method is to trace the area 

 through the lower stages and also through the older embryos. For the sake of 

 greater clearness, however, a description of the area will be given from its first 

 differentiation through its maximum transformation to its final disappearance for 

 the structure is only temporary. 



In pig embryos of 8 mm. and less in crown-rump measurement, the roof of the 

 fourth ventricle is formed of cells morphologically and tinctorially different from 



