A CONSIDERATION OF THE EMBRYONIC PIA MATER. 73 



the roof of the fourth ventricle is composed of the many-layered, epithelial-like cells. 

 This is well shown in a photomicrograph (fig. 53) from an injected human embryo of 

 9 mm. (No. 721) of the Carnegie collection. Likewise, in this region in a pig embryo 

 of 8 mm. (fig. 25), the same absence of a real pial condensation may be made out. 

 But this peculiarity of the pia is most striking at the period of maximal differen- 

 tiation of the superior membranous area in the rhombic roof. In figures 37 and 43, 

 photomicrographs from pig embryos of this stage, the mesenchymal condensation, 

 augmented by some vascular endothelium, is shown in adhesion to the ependyma 

 on both sides of the membranous area; but directly behind the differentiated cells 

 of the area membranacea evidence of a condensation of mesenchyme is wholly 

 lacking, even though both specimens show vascular channels in close approximation. 

 Similarly, in a human embryo of 14 mm. (No. 144, Carnegie collection) a total lack 

 of the true pial thickening is to be observed (fig. 57) . 



Quite similar is the failure of a pial thickening about the inferior membranous 

 area. This can be made out in figures 83 and 87, from human embryos in which 

 the process of differentiation of the area is proceeding. In later stages of the forma- 

 tion of the area membranacea inferior, the marked absence of a true pial condensa- 

 tion in the mesenchyme in this region is noted in figure 75 (a specimen from a fetal 

 pig of 23 mm.) But this apparent failure to form the typical mesenchymal conden- 

 sation of the pia mater in certain areas in the roof of the fourth ventricle must not 

 be construed as indicating an absence of pia mater. Such does not seem to be 

 the case here, for in the later stages of the formation of the cisterna cerebello- 

 medullaris the area membranacea inferior is found entirely unsupported, except for 

 a layer of mesenchymal cells. This is shown in figures 77 and 79, both taken from 

 fetal pigs of 32 mm. This mesenchymal layer must be considered as pia mater 

 apparently modified for a specific purpose. 



The general process, then, of formation of the pia mater concerns a condensa- 

 tion of mesenchymal elements to form an embryonic membrane about the central 

 nervous system. From its earliest beginning very slight modification is needed to 

 reduce it finally to the histological character of the adult membrane. The general 

 process holds, except in the regions of the arese membranaceae in the roof of the 

 fourth ventricle; here, apparently, a modification of the pia for a specific purpose, 

 involving an absence of the primary pial condensation, takes place. 



THE RELATION OF THE PIA MATER TO THE FLUID CHANNELS. 



The cerebro-spinal fluid in its normal pathways comes everywhere into contact 

 with the pia mater, which serves as the inner retainer for the subarachnoid space; 

 therefore the functional relation of this membrane to the fluid which bathes it 

 becomes of interest. To some degree the results of the experiments recorded in the 

 earlier portions of this paper throw light upon the relation of the pia mater to the 

 circulating fluid. The most important question in this connection is naturally that 

 dealing with the possible penetration of the normal fluid through this embryonic 

 membrane. In this regard the findings in replacement experiments with ferrocya- 

 nide solution serve to elucidate the problem. These observations give no evidence 



