GENERAL HISTOLOGICAL DIFFERENTIATION OF CEREBRO-SPINAL SPACES. 67 



This process of the breaking-down of the mesenchymal spaces to form fewer 

 and larger spaces goes on very rapidly in pig embryos as they exceed the length of 

 18 mm. Thus, figure 75 (from a pig embryo of 23 mm.) shows a marked decrease in 

 the mesenchymal elements about the medulla; the strands are becoming fewer in 

 number, and the albumen-filled spaces are increasing rapidly in size, but decreasing 

 in number. About the mesencephalon, however, the process has only just begun 

 (also shown by fig. 74) . In this photomicrograph (fig. 75) the mesenchymal elements 

 have broken down somewhat; the spaces are becoming enlarged, and a fine albumi- 

 nous coagulum fills the interstices between the mesenchymal processes. The whole 

 picture conveys an excellent idea of the forces which convert the many-spaced 

 mesenchyme into the much fewer cerebro-spinal channels. 



This general plan of the formation of the larger subarachnoid canals reaches 

 its maximum in the formation of the various cisterna? for cerebro-spinal fluid. The 

 process is probably best illustrated in the case of the cisterna magna, which persists 

 in the posterior cerebello-bulbar angle. Figures 74 and 75, taken from an embryo 

 pig 23 mm. long, give an idea of the initial formation of the cisterna cerebello- 

 medullaris. The mesenchymal strands, as shown in figure 75, are already broken 

 down in part, and are profusely covered with albuminous coagula. The process 

 has not proceeded to any extent in this specimen of 23 mm., but in the course of the 

 next 10 millimeters' growth extensive changes occur, as are shown in figures 76 and 

 77, photomicrographs from an embryo of 32 mm. In the space outside the inferior 

 membranous area the mesenchymal trabeculae have almost disappeared ; the space 

 or cistern, as it should now properly be called is almost completely filled with the 

 clotted albumen. The mesenchyme is seen running through this embryonic cistern 

 as a few isolated strands, but most of the tissue appears now as a fairly definite 

 membrane on the outer side of the space. This membrane will go to form the inner 

 surface of the dura and the continuous outer layer of the arachnoidea, as it furnishes 

 a visceral layer for the subdural space. 



More laterally in this same specimen the formation of the cistern has progressed 

 to an even greater extent. In figures 78 and 79 the total freedom of the lower portion 

 of the cistern from trabecular strands is seen; above, the mesenchyme still sweeps 

 down as a supporting structure for the chorioid plexus. A definite differential line 

 of mesenchymal condensation indicates the future outer border of the arachnoid as it 

 incloses the cisterna cerebello-medullaris. This general process of mesenchymal 

 breaking-down, altering the original small spaces into the larger arachnoid channels, 

 holds as the embryo develops into larger forms. 



In addition to this formation of the subarachnoid spaces in the adult through 

 the enlargement of the embryonic mesenchymal spaces, the perimedullary mesen- 

 chyme undergoes in these same localities condensations which result ultimately in 

 the formation of the arachnoid membrane and the trabeculae dividing up the cavum 

 subarachnoideale. Mention has already been made of the adhesion of the cell- 

 bodies of the disrupted mesenchymal elements to the persisting strands the initial 

 step apparently in the ultimate differentiation of the mesothelial cells which line 



