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



In the first place, it is conceivable that the cells in the differentiated areae mem- 

 branaceae could phagocyte the colloidal albuminous particles of the ventricular fluid 

 and excrete them into the subarachnoid spaces on the other side of the membrane; 

 but it does not seem probable that this explanation is correct. Much more likely is 

 it that the colloidal masses may follow the same laws of fluid-passage as the true 

 solutions. But in such a passage through a cellular membrane the rate of passage 

 will be much slower with the colloid. 



These two theories regarding the passage of the albumen of the ventricular 

 cerebro-spinal fluid into the subarachnoid spaces are not based on any findings 

 presented in this article, but are ventured as being in keeping with current physio- 

 logical explanations of such phenomena. On the basis of the second hypothesis, 

 the failure of granular material to pass through the cellular membrane of the chori- 

 oidal roof must be explained as being due to the inability of the cells to handle the 

 foreign material except in sizes which could be absorbed. The fact that the origi- 

 nal unit was not phagocyted or passed through the membrane probably depended 

 on the size of the molecule and the specific character of the lining-cells. 



THE PASSAGE OF SILVER NITRATE AND INDIA INK THROUGH THE MEMBRANOUS AREAS 

 IN THE ROOF OF THE FOURTH VENTRICLE. 



Thus far in the discussion of the passage of the experimental fluids through the 

 ventricular roof, true solutions of potassium ferrocyanide and iron-ammonium 

 citrate only have been considered. This solution, as has been pointed out in this 

 and in a previous article^ 55 ), is non-toxic and is not taken up by the cells. With the 

 dilute solutions (0.25 to 0.5 per cent) of silver nitrate, a far different problem is 

 presented. Replacement experiments with this salt are rendered impossible by its 

 intraspinous toxicity and by its precipitating action upon protein; but beautiful 

 preparations may be made by this method by the simple injection with a syringe 

 into the central canal of the spinal cord. 



With mild syringe-pressure the result of such an injection with silver nitrate is 

 in all cases a simple ventricular spread, with no extension into the periaxial tissues. 

 This general rule holds in all stages in which the central canal can be definitely 

 entered without causing a spread into the perispinal tissues. This failure of the 

 spread to extend into the periaxial tissues under mild pressure is undoubtedly due 

 to the coagulating effect of the silver, which renders further passage of the fluid 

 impossible. The reduced silver collects about the superior membranous area in the 

 roof of the fourth ventricle, outlining it distinctly. This phenomenon is illustrated 

 in figure 115 (a transverse section of a pig embryo of 19 mm.). At this stage the 

 replacement of cerebro-spinal fluid by a ferrocyanide solution results in a quite 

 extensive spread (cf. fig. 5). 



With increased pressures of injection the silver may be pushed into the periaxial 

 tissue through the roof structures of the fourth ventricle. The transit of the injec- 

 tion-mass occurs in the area membranacea superior in practically all cases (cf. fig. 12). 

 The inferior membranous area, in the earlier stages, is almost invariably impermeable 



