54 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



Could not these roughened, irregular cell-surfaces become the site of the first and 

 most extreme precipitation of the prussian-blue and of the reduction of the silver? 

 Certainly they would serve much more efficiently as the foreign substances about 

 which precipitation would occur in greatest amount. This physical explanation 

 finds many arguments for its support in these studies. 



Another explanation of the phenomenon concerns the normal flow of the fluid 

 and the relation of the direction of this flow to the roof of the fourth ventricle. As 

 has already been emphasized, it is difficult to assume that there is any marked pro- 

 duction of cerebro-spinal fluid before the periaxial spread occurs. Such an assump- 

 tion would argue against the development of any special current toward the roof of 

 the fourth ventricle in any stage smaller than that represented in figure 3, and would 

 vitiate the explanation of the occurrence of the granular accumulation shown in 

 figure 2 (a pig embryo of 13 mm.). In the later stages (16 mm., cf. fig. 11) this 

 explanation would probably suffice for the phenomenon exhibited. 



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



With consideration of the evidence presented as to the accumulation of the 

 precipitates of the injected fluid about the area membranacea superior during 

 certain stages in the development of the cerebro-spinal spaces, it would seem that 

 the same area must be concerned in the passage of fluid from the ventricular cavities 

 into the periaxial tissues. This view receives support from the reproduction of a 

 cleared specimen (fig. 11) in which an injection of silver nitrate had been made into 

 the central canal of the spinal cord. The pressure employed was great enough to 

 force the fluid into the periaxial spaces, but the resultant picture clearly showed the 

 oval outline of the area membranacea superior. 



The study of the passage of fluid from the ventricular to the extraventricular 

 spaces can best be made by simple histological serial sections. In these observations 

 pig embryos in which the cerebro-spinal fluid had been replaced by the compensating 

 device, supplying a true solution of potassium ferrocyanide and iron-ammonium 

 citrate, were sectioned and examined with reference to the sites of fluid passage. 

 The results of these studies are given here in order that the whole question of the 

 connection of the cerebral ventricles with the subarachnoid spaces may be discussed. 



In the stage represented by figure 3 (in which fluid passes from one area in the 

 roof of the fourth ventricle into the extraventricular tissues) histological sections 

 show that the point of fluid passage is localized and concerns solely the area mem- 

 branacea superior. The replaced fluid (as demonstrated by the subsequent precipi- 

 tation of the prussian-blue) passes through this entire membranous area into the 

 adjoining mesenchyme. The process is wholly confined to this area; the adjoining 

 ependyma is entirely impervious to the ferrocyanide. This phenomenon of passage 

 of the replaced fluid through the superior membranous area is well shown in figures 

 14, 18, and 23. 



The distribution of the minute granules of prussian-blue in the cells of the 

 superior membranous area is of importance in any discussion of the passage of fluid 

 through a membrane; for this area (in the superior portion of the roof of the 



