62 DEVELOPMENT OF CEREBRO-SPINAL, SPACES IN PIG AND IN MAN. 



The most important question in this connection is whether the ependymal 

 differentiation is necessary for the passage of fluid through it. In the pig embryo 

 of 13 mm. the area membranacea superior has reached a stage of marked differ- 

 entiation (fig. 31), but at this same stage (fig. 2) there is no evidence of any passage 

 of fluid through the roof of the fourth ventricle into the periaxial tissue, only an 

 outlining of the oval membranous area. Here, then, the histological differentiation 

 has definitely preceded the assumption of function on the part of the area mem- 

 branacea superior. The passage of fluid through the lower area occurs at a rela- 

 tively earlier stage than it does through the superior opening. The first evidence of 

 differentiation of the inferior roof of the fourth ventricle was observed in pig 

 embryos of 15 mm. in length. At 18 mm., even though the process of differen- 

 tiation was far from complete, some of the replaced fluid was able to pass through 

 the lower area (figs. 4, 16, and 18). 



A consideration of these observations leads to the assumption that some histo- 

 logical differentiation of the ependyma is necessary for the extraventricular passage 

 of the replaced fluid. In the case of the superior area the differentiation occurs at 

 a considerable developmental interval before fluid passes through it; in regard to 

 the inferior area the assumption of function occurs at a somewhat earlier period in 

 its differentiation. This slight difference between the two areas may possibly be 

 explained on the basis that as soon as the stage of 14 mm. is attained (by the pig 

 embryo) a greater amount of cerebro-spinal fluid is produced than can be cared for 

 by the more slowly enlarging ventricular cavities. As soon as this disproportion 

 occurs the excess of fluid is poured into the periaxial tissues through the already 

 differentiated area membranacea superior; therefore, when the inferior area first 

 shows evidence of formation there is still this excess of fluid in the ventricles. The 

 fluid apparently avails itself almost at once of the new opening and its functional 

 existence becomes immediate. It is apparent, moreover, that the capacity of the 

 membranous areas for the passage of fluid is considerably in excess of the demands 

 made upon them, and furthermore, that the provision for the passage of increasing 

 amounts of fluid is completed before the demand arises. 



In the passage of fluid from the ventricles into the mesenchyme, there is one 

 other factor which has not as yet been considered. This concerns the potentiality 

 of the adjacent mesenchyme to afford channels for the fluid poured into it. Were 

 resistance offered to the flow of solutions through the mesenchymal tissue spaces, 

 fluid could escape from the ventricles in only very small amounts, if at all; as soon, 

 however, as easily traversed fluid channels became established, the cerebro-spinal 

 fluid could readily escape through the two membranous areas. The question as to 

 what part the embryonic cerebro-spinal fluid plays in the further development of the 

 meningeal spaces also arises in this connection. It is at present impossible to assign 

 to any one of these factors a specific role in the passage of fluid from the fourth 

 ventricle into the periaxial spaces, but it is important to consider them as possible 

 determining agents. The evidence all indicates that the rate of production of the 

 embryonic cerebro-spinal fluid is the most important factor, by far, in the extra- 

 ventricular escape of the fluid. 



