PASSAGE OF FLUID THROUGH ROOF OF THE FOURTH VENTRICLE. 53 



VI. PASSAGE OF FLUID THROUGH ROOF OF THE FOURTH VENTRICLE. 



On pages 20 to 30 is a description of the passage of a true solution, substituted 

 without increase in pressure for the embryonic cerebro-spinal fluid, through the 

 roof of the fourth ventricle into the extraventricular or periaxial spaces. This 

 extension of fluid occurred in two localized areas, one in the superior half and the 

 other in the inferior half of the rhombic roof. Histological study of these regions 

 revealed a localized differentiation of the ependyma, both in the upper and lower 

 halves of the ventricular roof. It becomes necessary, then, to correlate, if possible, 

 the areas of this fluid-passage to the anatomical differentiations pointed out. 



THE ACCUMULATION OF INJECTION-MASSES IN THE SUPERIOR MEMBRANOUS AREA. 



It has already been recorded that the first evidence of a change in the reaction 

 to a replacement injection occurred in an embryo about 13 mm. long (fig. 2). This 

 stage was characterized by a dense collection of the precipitated granules in a 

 definite area in the roof of the fourth ventricle. At this stage also the area mem- 

 branacea superior is well differentiated (fig. 31). That the site of the granular 

 accumulation is this membranous area is easily proved by an inspection of figure 

 117, which represents an enlargement of the squared area in figure 116. In the low- 

 power photomicrograph the prussian-blue granules are not represented, but are 

 found scattered through the ventricles, with a definite collection in the posterior 

 region of the fourth ventricle. Under a higher magnification (fig. 117) the blue can 

 be traced in but small quantity along the normal ependymal lining (shown to the 

 left in the figure) , but as soon as the differentiated area (area membranacea superior) 

 is reached the granular material is heaped up in a dense mass, which extends as a 

 thickened pad into the ventricle. 



The same phenomenon of the accumulation of the injection fluid in the superior 

 membranous area is shown in figures 112 and 113, the second photomicrograph 

 representing the area outlined in the first, but reproduced under much higher 

 magnification. In this specimen (an embryo pig) a dilute solution of silver nitrate 

 was injected into the central canal of the spinal cord. On histological examination 

 the accumulation of the silver also shown in figure 11 was found. Thus, in figure 

 113 the ventricular epithelium can be made out in the upper right-hand corner, while 

 below (in the area membranacea superior) the silver is densely accumulated. 



The explanation of this phenomenon of accumulation in the superior membra- 

 nous area is not wholly clear. It occurs only in stages in which the histological 

 differentiation of the ventricular roof has proceeded to some degree and in stages 

 where the fluid-passage into the periaxial tissues is not wholly unobstructed. This 

 aggregation of the precipitated granules of prussian-blue and of the reduced silver 

 in a localized area certainly suggests a physical explanation, as in these cases the 

 physical laws of precipitation and reduction must hold. The many figures of the 

 superior membranous area of the ventricular roof show that in the stage under con- 

 sideration the cell-outlines projecting into the ventricles are rough and ragged as 

 contrasted with the smoother and more regular surface of the adjoining ependyma. 



