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



the introduction of specialized arachnoidal cell-chains running throughout the 

 pachymeninx. Unquestionably, the cerebro-spinal fluid possesses its own peculiar 

 and characteristic pathway, analogous in no way to the lymphatic vessels of other 

 tissues. 



The outer continuous membrane of the arachnoidea forms as a mesenchymal 

 condensation, at first in common with the inner surface of the dura mater, but soon 

 separated from it by the subdural space. The very low cubical mesothelium which 

 covers the arachnoid membrane on both surfaces and also invests the arachnoid 

 trabeculae differentiates apparently from the original mesenchymal elements in the 

 periaxial tissues. 



One of the most interesting features of this study has been the relation of the 

 various mesenchymal condensations to the foreign true solution which was intro- 

 duced into the medullary-canal system. This fluid circulated throughout the peri- 

 axial spaces which enlarge to form the subarachnoid channels, but it never pene- 

 trated the primary blastema which served as a primitive dura, nor did it ever invade 

 the pial cells which so closely adhere to the nervous tissue; likewise, as soon as the 

 secondary mesenchymal condensation dividing the dura from the arachnoid spaces 

 appeared, this condensation was impervious to the true solution. No evidence of 

 any penetration, as might be expected as due to diffusion, could be made out. 



This summary has been included in order that some correlation between the 

 topics discussed separately in the foregoing sections might be made. No attempt 

 has been made here to present the findings in abstract form; these have been sum- 

 marized at the end of each division of this communication. 



