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



The same phenomenon of the passage of fluid from the fourth ventricle into the 

 periaxial spaces is beautifully illustrated in figure 23. This drawing is from a trans- 

 verse section of a pig embryo (23 mm. in length) in a stage when the superior mem- 

 branous area is rapidly being encroached upon by the developing cerebellum and by 

 the caudal chorioid plexuses. Between the deeply staining ependymal cells on 

 either side the membranous area is densely outlined by the deposition of the granules 

 of prussian-blue in the cytoplasm of the cells of the area membranacea superior. 

 The avoidance of the nuclei of these cells by the ferrocyanide is well demonstrated 

 in this reproduction, as is also the impenetrability of the ependymal cells. In a 

 specimen of this nature the question of the passage of the injection fluid through 

 possible intercellular foramina loses its significance; for the drawing shows clearly 

 the importance of considering the entire area membranacea as a functioning whole 

 a permeable, living, cellular membrane. 



It has been shown in a foregoing section of this memoir that histologically 

 the area membranacea superior decreases to an almost negligible remains in speci- 

 mens of embryo pigs over 30 mm. long. This same rule apparently holds for its 

 functional importance, as determined by the relative and absolute amount of 

 prussian-blue granules deposited in the cells of the superior membrane. This 

 decrease in the functional importance may be inferred from figure 47, a photomicro- 

 graph from a pig embryo of 32 mm. Apparently the size of the membrane deter- 

 mines in large measure the amount of the replaced fluid which passes through it. 



Thus far we have been concerned solely with the passage of fluid through the 

 area membranacea superior. In the earlier stages of from 14 to 23 mm. the 

 importance of the superior membrane functionally is great, but in the later stages 

 the inferior membrane assumes far greater significance. This is demonstrated not 

 only by the structural history of the two areas, but by the functional index afforded 

 by the replacement of the cerebro-spinal fluid by a foreign solution. 



In the foregoing section the first evidence of any histological differentiation in 

 the inferior portion of the roof of the fourth ventricle was shown to occur in pig 

 embryos of 15 mm. in length. From this stage upwards (figs. 4, 5, etc.) a portion 

 of the inferior roof allows fluid to pass through it. The exact point of fluid passage 

 is the localized ependymal differentiation forming the area membranacea inferior. 

 This relationship is easily verified by reference to figure 18. In this drawing of a 

 median sagittal section of a pig embryo the two localized points of fluid passage into 

 the periaxial tissue are readily identified; they are quite limited in comparison to 

 the extent of the periaxial spread. 



Figure 16 represents the inferior membranous area of the roof of the fourth 

 ventricle from a pig embryo of similar size (18 mm.). The histological character 

 of the inferior area is well shown in this drawing. It will be seen that, except in 

 small areas, the histological differentiation of the ependyma has not proceeded to any 

 great extent; the fluid from the ventricular cavity (as traced by the precipitated 

 granules) closely follows the points of greatest cellular differentiation. There is 

 no possibility of an interpretation of the findings concerned with the existence of 



