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



formation before the tissue fully differentiates into the adult arachnoid membrane. 

 Strangely enough, a similar collection of an embryonic type of tissue is sometimes 

 met with, in these stages, between the two hemispheres. 



The general process, then, of the formation of the arachnoidea involves both 

 a breaking-down (or thinning-out) of the mesenchymal spaces and a condensation 

 of the cells. The first of these processes results hi the transformation of the inter- 

 stices of the periaxial mesenchyme into the larger subarachnoid spaces, divided off 

 by arachnoid trabeculae; the second finds its final accomplishment in the develop- 

 ment of the outer arachnoid membrane which, covered with mesothelial cells, forms 

 the inner surface of the subdural space. The transformation begins in the basilar 

 regions of the cranium and spreads upward over the hemispheres. 



THE CIRCULATION OF FLUID THROUGH THE SUBARACHNOID SPACES. 



In view of the processes of differentiation involved in the formation of the 

 arachnoidea and the subarachnoid spaces, the circulation of fluid through this 

 peculiar membrane must be considered. It seems important to ascertain, if possible, 

 the relationships between the beginning of the passage of the cerebro-spinal fluid 

 and the onset of the histological changes. 



The conceptions of the development of the circulation of the cerebro-spinal 

 fluid which are presented in this communication are dependent, in large measure, 

 upon the results of the replacement of the fluid, in living embryos, by the ferro- 

 cyanide solution. Additional evidence was obtained from the identification of 

 albuminous coagula in the periaxial tissues. The correlation of these findings with 

 the development of the chorioid plexuses and with the results of injections under low 

 pressures, from a syringe and so forth, gave evidence of their correctness. 



The differentiation of the mesenchyme into arachnoid membrane may be said 

 to keep pace with the establishment of the periaxial channels for the cerebro-spinal 

 fluid. In the main, the passage of this fluid into the undifferentiated mesenchyme 

 about the nervous system precedes the process of histological change. This phe- 

 nomenon is shown in figure 14, from a pig embryo of 18 mm. The replaced fluid 

 is seen passing out into the mesenchyme through the two membranous areas in the 

 roof of the fourth ventricle. The mesenchyme at this stage has already differen- 

 tiated somewhat, but hardly in proportion to the length of time during which the 

 fluid has been passing into the space. 



There are several features of interest in the course of the fluid through the peri- 

 axial spaces. In sections of embryos in which the cerebro-spinal fluid has been 

 replaced by a foreign solution the granules of the precipitated salts may be identi- 

 fied in the periaxial mesenchyme in situations corresponding exactly to the extent 

 of the spread shown in the cleared specimens (figs. 1 to 9). The exact location of 

 the prussian-blue granules is of importance in this connection, as the exact form and 

 distribution of the periaxial spaces and their relation to the adult subarachnoid 

 spaces may thus be determined. 



Examination of serial sections from an embryo in which the embryonic ventric- 

 ular fluid has been replaced by the ferrocyanide will reveal, if the embryo exceeded 



