90 DEVELOPMENT OF CEEEBRO-SPINAL SPACES IN PIG AND IN MAN. 



of absorption on the part of the cerebral veins and embryonic capillary plexuses. 

 In the adult, however, the process is not diffuse, but is confined to the arachnoidal 

 villi, while in the embryo a considerable extent of the inner wall of the sinus lying 

 in the mesenchymal tissue, which is breaking down to form the arachnoidal spaces, 

 serves as a site for the fluid passage. In these earlier stages the sinus transversus 

 functions as the chief sinus of absorption. This is probably to be explained by the 

 primary basilar spread of the replaced cerebro-spinal fluid and also by the fact that 

 the true sinus sagittalis superior is a much later addendum. In the human embryo, 

 according to Streeter( 54 ), it is found in stages of over 50 mm. 



The absorption of cerebro-spinal fluid in the embryo seems to follow the direct- 

 ing agencies which operate in the adult. Increase in the pressure employed in the 

 injection of true solutions results in the drainage of more of the fluid, as determined 

 by subsequent microscopical examination. This suggests that the process is deter- 

 mined by factors other than that of diffusion; it seems most likely that here, too, 

 the process is one of nitration, with a possible distension of the cellular membrane, 

 so that intercellular spaces are opened. The histological picture of the sinus wall, 

 however, undoubtedly gives the impression that the fluid has passed almost solely 

 through the cytoplasm of the endothelial cells and likewise through the layer of sup- 

 porting mesenchyme. These findings are in accord with observations on the adult. 



With dilute suspensions of india ink as the injection mass, the results are quite 

 different in regard to the passage of the material into the sinus. Replacement 

 experiments making use of this suspension of particulate matter yield no evidence, 

 as the carbon granules do not leave the ventricular system. Likewise, simple injec- 

 tions of the suspension into either the central canal of the spinal cord or into the 

 perispinal spaces furnish no information unless the syringe-pressure be high. In 

 this case the carbon granules may be traced into the sinus transversus, which is 

 apparently the point of least resistance. Because of the obscuring of the picture by 

 the carbon it can not be determined histologically whether the granules pass into 

 the sinus in the same manner as does a true solution, or whether the passage is 

 effected by numerous small ruptures of the tissue. The impression gained from our 

 study would incline one toward the latter view. 



If the injection of india ink be made under very great pressure from a syringe, 

 the segmental veins may be filled with the carbon. This filling is always subse- 

 quent to its flow into the sinus transversus. But in no case was an evidence of a 

 flow into lymphatic channels observed. 



The process of drainage of the cerebro-spinal fluid into the venous system of 

 fetuses will not be detailed here. This undoubtedly concerns a study of the forma- 

 tion of arachnoidal villi and of the differentiation of the lateral walls of the superior 

 sagittal sinus, the best site for this study. The material at hand is not suited for 

 this investigation, so that postponement is necessary. 



