A CONSIDERATION OF THE EMBRYONIC PIA MATER. 71 



14 mm. in length, granules of prussian-blue in the peribulbar mesenchyme (fig. 14). 

 The granules are not found in any cell-bodies in this tissue; they are made out, in 

 large measure, adhering to the mesenchymal cell-processes or lying free in the mesen- 

 chymal interstices. The granules do not penetrate the pia mater or the dura 

 mater, a finding which will be discussed more fully in the sections dealing with these 

 membranes. Everywhere the transit of fluid into the nervous tissue seems to be 

 prohibited by the pia; in some areas, however, the outer condensation of mesen- 

 chyme to form the dura-periosteum has not yet occurred. This is shown particu- 

 larly well hi the region of the roof of the fourth ventricle (fig. 18), where the epi- 

 dermis offers the only barrier to the passage of fluid from the pericerebral spaces. 



In the earlier stages in which the phenomenon of fluid passage about the central 

 nervous system may be observed, the outer layer of the arachnoid is not at all 

 differentiated. Here the barrier to the fluid is the blastemal condensation of mesen- 

 chyme (fig. 16). In the later stages, when the outer layer of the arachnoid is begin- 

 ning to appear as a mesenchymal thickening, the fluid (as indicated by the precipi- 

 tated prussian-blue) is confined strictly within the immature arachnoid membrane. 



The course, then, of the fluid which has replaced the cerebro-spinal fluid in the 

 embryo follows that of the adult cerebro-spinal fluid (as shown by the resultant 

 blue granules). It is everywhere contained within spaces which topographically 

 and embryologically correspond to the subarachnoid spaces in the adult. The 

 spread of the replaced solution from the embryonic ventricle into the peribulbar 

 tissue is analogous in every way to the passage of cerebro-spinal fluid from the fourth 

 ventricle of the adult into subarachnoid spaces. 



VIII. A CONSIDERATION OF THE EMBRYONIC PIA MATER. 



Our present conceptions of the embryology of the pia mater are largely due to 

 the work of His^ 25 ) and of Kolhker^ 31 ). who first firmly established the idea that this 

 inner leptomeninx was mesodermal in origin. While generally accepted (Farrar^ 16 )) , 

 this view has not been widely referred to in the literature; but the absence from all 

 embryologies of any information concerning the development of the meninges is 

 quite striking and it does not seem strange, therefore, that our information regarding 

 the pia mater has not advanced in keeping with our knowledge of the embryology of 

 other structures of the body. In the present section of this communication it is 

 purposed to present merely a general consideration of the process by which the pia 

 mater is formed and to point out some of its functional characteristics, especially 

 in regard to the fluid channels. 



The term pia mater is accepted throughout this article as designating solely the 

 cellular membrane which adheres closely to the outer surface of the nervous system, 

 but it is in direct connection with the arachnoidal trabeculae which traverse the sub- 

 arachnoid space. Whether the two membranes should be considered together as 

 the pia-arachnoid or as the leptomeninx is a question in regard to which there is 

 some disagreement; it will suffice to consider the pia as a separate membrane. 



