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



noid spaces extending from pial to blastemal condensation. When in these embryos 

 the cerebro-spinal fluid is replaced by the ferrocyanide solution and the embryo 

 kept alive for some time, the course of the injection may be traced to varying extents 

 throughout the periaxial tissue. To this spread of the injection fluid (a true solu- 

 tion, during the progress of the experiment), however, the blastemal condensation 

 of mesenchyme opposes an absolute barrier. This peculiarity of the early conden- 

 sation may be readily seen in figures 16 and 18. At this stage in development the 

 blastemal thickening may be said to play the role of the outer membrane of the 

 arachnoidea or of the inner surface of the dura. 



This feature of the blastema as an impenetrable membrane an absolute 

 barrier to the passage of fluid is found also to endure during injections of the ferro- 

 cyanide solution under pressures sufficient to rupture other parts of the central 

 nervous system. Similarly, the early blastemal condensation resists the inflow of 

 the other injections used (india ink and silver nitrate) under similar pressure condi- 

 tions. In later stages the injection solutions, from ventricular or subarachnoid 

 introduction, do not reach the dura. This is due to the development of an outer 

 membrane of the arachnoidea and the formation of the subdural space. The 

 arachnoid membrane when formed does not permit fluid to pass outward into the 

 subdural space; but the competency of the early blastemal condensation in the 

 mesenchyme affords a very good example of the perfect function of a tissue as a fluid- 

 barrier. 



An interesting phase of the competency of the secondary mesenchymal con- 

 densation (forming dura and outer membrane of arachnoid) may be seen in the 

 region of the cisterna cerebello-medullaris. Here, as shown in figure 77, the zone of 

 secondary condensation, while complete below, does not remain definitive above as 

 the mesenchyme sweeps inward to the chorioid plexuses. At such a stage of 32 mm. 

 in the pig, a replacement experiment would show no penetration of this secondary 

 dural condensation by the foreign solution, where the condensation made a definitive 

 membrane; above, however, in the region of the plexuses, a limited penetration by 

 the introduced fluid could be made out. 



X. THE RETURN OF CEREBRO-SPINAL FLUID TO THE VENOUS SYSTEM. 



The question of the exact mode of return of the cerebro-spinal fluid to the general 

 circulation has interested many investigators. It has occasioned a large amount 

 of work, with the presentation of several hypotheses. Key and Retzius (29 ), from the 

 results of injections of colored gelatin into the spinal subarachnoid space, held that 

 the cerebro-spinal fluid returned through Pacchionian granulations into the great 

 dural sinuses. Other workers, following Key and Retzius, were dissatisfied with this 

 theory, because of the apparent lack of these granulations in infants and in the lower 

 animals. CathelinW, with but little evidence, hypothecated an absorption of the 

 fluid by way of the perineural sheaths into the lymphatic system, although the physi- 

 ological findings of Ziegler^ 57 ), Reiner and Schnitzler( 48 >, Leonard HilK 24 ), and others 

 made it necessary to consider a direct absorption into the blood system. Gushing W 



