GENERAL SUMMARY. 103 



basilar pericerebral spaces. Thus, the spinal spaces must be considered as develop- 

 ing physiologically from above, and not from below upward, as Reford found. The 

 complete filling of these perispinal spaces is found in pig embryos of 21 mm. At 

 this stage the pericerebral spaces are filled, with the exception of those around the 

 superior portion of the midbrain and about the cerebral hemispheres. 



The final filling of all the periaxial spaces occurred in pig embryos of about 

 26 mm. This phenomenon may be taken to indicate the establishment of the true 

 cerebro-spinal relationships of the adult, for in this case there is an intraventricular 

 production of the fluid and an extraventricular spread. Likewise, the fluid returns 

 to the venous system in embryos of over 23 mm., and this escape of the fluid from its 

 periaxial bed is, as in the adult, directly into the venous sinuses of the dura mater. 



The rapidity of the further extension of the replaced solution after the stage of 

 18 mm. is passed is apparently due to a second marked acceleration in the rate of 

 production of the ventricular cerebro-spinal fluid. As in the first instance, this 

 increased elaboration seems connected ultimately with the formation of the chorioid 

 plexuses of the third and lateral ventricles. As soon as these tufts develop, the 

 cerebro-spinal fluid is produced in amounts which far exceed the quantities for which 

 the more slowly enlarging ventricles can provide. 



The histories of the two area? membranacese of the fourth ventricle are dissimilar. 

 Both are areas apparently differentiated from the normal lining ependyma for a 

 specific functional purpose the passage of fluid from the ventricles into the future 

 subarachnoid spaces. The superior membranous area reaches its maximum func- 

 tional importance in the stages of 18 to 20 mm. in the pig and also in the human 

 embryo and from these stages on it slowly regresses. The final obliteration of the 

 area, if it do not persist as an occasional small remnant, is due to the increasing 

 growth of the cerebellum and the enlargement of the chorioid plexuses of the fourth 

 ventricle. On the other hand, the inferior membranous area continues to increase 

 both in size and functional importance after its initial differentiation from the 

 ependyma; it finally occupies the greater portion of the velum chorioidea inferior. 

 These observations can not solve the interesting question of a perforation of the 

 inferior velum to form the foramen of Magendie. 



Of the factors which influence the passage of fluid outward into the periaxial 

 spaces, it must be realized that probably there is difference in this regard between 

 the true solutions of the salts and the colloidal suspensions. For the true solutions 

 (as in the experimental replacements) diffusion probably plays some role; but that 

 this is not the sole factor is shown by the failure of the fluid to pass through the 

 membrane in the stages under 14 mm. The findings of the granules of prussian-blue 

 within the cytoplasm of the cells of this membrane indicates that the fluid passage is 

 similar in every way to that through a true membrane. There is also a possible 

 site of fluid passage between the cells of this membrane. But, surely, the most 

 important factor in this process is one of filtration of the fluid from the point of 

 higher pressure to one of lower. This is indicated by all of the findings: that the 

 increased production of the fluid or the increased intraventricular pressure (whether 



