PASSAGE OP FLUID THROUGH ROOF OF THE FOURTH VENTRICLE. 55 



embryonic fourth ventricle) must be considered as a membrane permeable in certain 

 degrees to the fluids bathing it. That the area membranacea is intact and does not 

 contain stomata or other minute foramina has been demonstrated histologically. 

 Further evidence of the entire lack of intercellular stomata is afforded by the distri- 

 bution of the prussian-blue granules precipitated in situ after the replacement of the 

 cerebro-spinal fluid by the ferrocyanide solution. 



Figure 14 is a reproduction of the superior area from a transverse section of a 

 pig embryo in which the routine replacement had been made. The position of the 

 area is shown by the squared outline in figure 13. On both sides the impermeable 

 ependyma is seen, with granules of the blue adhering to the ventricular border of 

 the cells, but not penetrating them at all. To the left of the drawing the few epen- 

 dymal cells possess, beneath their central border, a chain of the granules which have 

 entered from the abrupt edge of the area membranacea. In the cellular border 

 between the two lips of the ependyma, the area membranacea superior, the passage 

 of the replaced fluid is easily made out by the resultant blue granules. The area 

 is roughly delimited by a ventricular collection of the blue granules. Examination 

 of these cells shows that the prussian-blue is present within the cytoplasm, avoiding 

 the nuclei with perfect precision. Some of the cells are rounded and almost free 

 from the granules; others, particularly those whose cytoplasm is elongated, are 

 completely filled with the granules, the nuclei standing out in a blue granular 

 cytoplasm. 



The question of the passage of the fluid between the cells must also be answered 

 by the histological evidence. In the same drawing (fig. 14) in one or two places 

 there are indications of a slight stream of granules between the cells of the area 

 membranacea superior. This apparent transit of the fluid through intercellular 

 passages is particularly clear in the small areas where the cellular cytoplasm is 

 relatively free from the granular deposits. But upon careful examination of these 

 areas under oil immersion it is always apparent that the adjoining cytoplasm is 

 also involved in the granular precipitation, indicating that the cells, although almost 

 free from the deposit, are also engaged in the process of the fluid passage. Com- 

 pared to the whole area of fluid transit, the points indicative of a passage through 

 possible intercellular stigmata are almost negligible. It seems not unlikely that the 

 outlining of canals between cells may be a physical phenomenon, as in most cases 

 no cellular borders (as demonstrated by the precipitated granules) can be made out. 

 These peculiarities of fluid passage may be seen in figures 14, 18, and 23. 



Consideration of all the evidence afforded by histological examinations of the 

 essential character of the area membranacea superior and of the passage of fluid 

 through it inclines one inevitably to the belief that this area functionates as a 

 cellular membrane. The fluid passes through it as through any permeable living 

 membrane. Histologically the passage is for the most part through the cytoplasm 

 of the cells, but occasionally an intercellular course is suggested. Both processes 

 are wholly compatible with the accepted view of a cellular membrane devised for 

 the passage of fluid through it. 



