PASSAGE OF FLUID THROUGH ROOF OF THE FOURTH VENTRICLE. 57 



intercellular stomata; the passage of fluid is here again to be looked upon as a 

 transit through a cellular membrane. 



The same general phenomena of the passage of fluid through a localized area 

 (the area membranacea inferior, in the caudal portion of the roof of the fourth 

 ventricle) that have been observed in the superior portion of the roof are shown in 

 figure 18. Chief among these phenomena is the careful avoidance by the precipitated 

 granules of the ependymal lining of the ventricles and the adherence of the granules 

 to the lining walls at the points of fluid passage. The ependymal lining, except in 

 the two areas of differentiation, is everywhere impenetrable to the solution of the 

 ferrocyanide. 



As the size of the embryo increases the functional importance of this more caudal 

 area becomes much greater (cf. figs. 3, 4, 5, and 76). The whole caudal half of the 

 fourth ventricle becomes an area of ependymal differentiation and of fluid passage. 

 It serves everywhere as a complete diffusing membrane, unbroken by the occurrence 

 of stomata. Through this whole membrane the replaced solutions of potassium 

 ferrocyanide and iron-ammonium citrate pass with apparent ease, as demonstrated 

 by the precipitated granules of prussian-blue (fig. 18). From stages of 24 mm. and 

 over, the lower membranous area is the only one of significance in the total fluid 

 passage. 



The areas, therefore, through which the replaced solution of potassium ferro- 

 cyanide and iron-ammonium citrate passed, in the experimental pig embryos, are the 

 two areas of histological differentiation in the roof of the fourth ventricle the area? 

 membranacea? superior et inferior. There is no evidence whatsoever of any other 

 point of escape of the fluid from the ventricular system into the periaxial spaces. 

 The precipitated prussian-blue does not penetrate any of the lining cells of the ven- 

 tricle except in the two areas under consideration. Nor is any evidence afforded 

 by histological study of the escape of ventricular fluid through the described fora- 

 mina of Bichat and of Mierzejewsky. 



FACTORS CONCERNED IN THE EXPERIMENTAL FLUID PASSAGE. 



It becomes necessary to discuss the question of the passage of the replaced fluid 

 through the two cellular membranes in order to ascertain to what extent the results 

 obtained by the method may be relied upon. Naturally in such questions the 

 factors concerned in the normal transit of body-fluid through such structures must 

 be considered. 



Probably the most essential element in obtaining reliable results in any injec- 

 tion is the control of the pressure at which the foreign fluid or mass is introduced. 

 This matter has been fully discussed in the resume of the methods employed; it is 

 sufficient to reaffirm here that, in these observations, the normal cerebro-spinal 

 tension has not been disturbed because of the use of a compensatory replacement. 

 Other experiments, carried out under increasing pressures of injection, have been 

 made, in order to compare the results with those furnished by the replacement- 

 method. 



