PASSAGE OF FLUID THROUGH ROOF OF THE FOURTH VENTRICLE. 59 



in figure 2 one would expect as extensive a spread of the replacing solution into the 

 periaxial tissue were diffusion the active force in the movement of the fluid. Instead 

 of such a periaxial spread the injection fluid remains wholly within the ventricular 

 system, indicating that other forces than that of diffusion play an active role, in 

 the more advanced stages, in the movement of the fluid. Finally, if diffusion is to be 

 considered the sole agent in the distribution of the replacing fluid, why does not the 

 ferrocyanide penetrate all the cellular structures lining the ventricular cavity? 

 Surely it would be expected that diffusion between the body-fluids and the ferro- 

 cyanide solution would occur in each ependymal cell a phenomenon observed only 

 in the cells comprising the ventricular surfaces of the membranous areas of the 

 rhombic roof. 



While acknowledging that diffusion and osmosis may play important parts in 

 the process of the passage of fluid from the fourth ventricle into the periaxial tissues, 

 it seems apparent that some other factor or factors must be the determining agent or 

 agents. It is not unlikely that the formation of cerebro-spinal fluid by the cells 

 of the chorioid plexus may cause, in the replacement experiments, further passage 

 of fluid into the extraventricular regions. Such an elaboration of fluid, with the 

 ventricles filled with the experimental solution, would result in an increase in the 

 normal ventricular tension. If this be the real explanation, the passage of the fluid 

 into the extraventricular spaces would result in part from the increase in pressure 

 on one (the ventricular) side of the membrane. The process, then, would be one 

 of filtration through the membrane from the point of higher to that of lower pressure. 

 This explanation best seems to cover the results obtained by the replacement 

 method, and is supported by the histological examination of the developing chorioid 

 plexuses and by many other features which are dealt with in other sections of the 

 paper. This view is also strongly supported by the results of injections under mild 

 syringe-pressure. 



On the basis that the passage of fluid from the fourth ventricle into the periaxial 

 tissues is in large measure a process of membrane filtration, the phenomenon of the 

 fluid transit of the replaced solutions may be taken as a real index of the circulation 

 and distribution of the cerebro-spinal fluid. It may be assumed, therefore, that the 

 resulting distribution of the prussian-blue granules represents the course and extent 

 of the fluid channels of the embryonic cerebro-spinal fluid. 



The discussion of the fluid passage outward from the cerebral ventricles into 

 the subarachnoid spaces has thus far been concerned with the processes involved 

 for the transit of the true solutions of the salts. There is, however, an undoubted 

 passage outward, as has already been indicated in a foregoing section, of the pro- 

 tein content of the normal cerebro-spinal fluid. This occurs in specimens in which 

 a truly definitive membrane, intact throughout, can be seen inclosing the chorioidal 

 roof. The explanations which suffice for the passage outward of the true solutions 

 will not serve for this phenomenon. 



The cells of the body probably are equipped to handle colloidal solutions in 

 several ways, but two methods seem possible as explanatory of the problem at hand. 



