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



these spaces. Gradually with the increasing growth of the embryo these cells 

 seemingly become arranged in definite columns covering the persisting arachnoidal 

 trabeculae. At the same time a differentiation of these primitive mesenchymal 

 elements occurs, the cells ultimately being transformed into the very low cuboidal 

 mesothelium of the subarachnoid spaces. This differentiation begins first in the 

 basilar portions of the cranium and spreads upward, in a way similar to the course 

 of development of the cranium and of the enlargement of the pericerebral spaces. 



While such a general process as outlined accounts for the formation of the 

 arachnoidal trabeculse and the subarachnoid spaces, it has but little bearing on the 

 development of the outer intact membrane of the arachnoidea. This portion of 

 the arachnoidea (which might be termed the arachnoid membrane as distinguished 

 from the arachnoid trabeculse) first appears as a distinct line of mesenchymal con- 

 densation separating the mesenchyme into the primitive arachnoid and dura mater, 

 as in figures 76 and 77, dmc. This rather thin zone of cellular density in reality 

 represents not only the outer surface of the arachnoidea, but also the inner surface 

 of the dura mater. At first these develop in close fusion with a later separation of 

 the two membranes. With this cleavage of the two surfaces, the arachnoid mem- 

 brane rapidly differentiates, forming an intact layer over the subarachnoid spaces. 

 The cells covering the surface membrane seem to change gradually into the low 

 cuboidal type, similar to those covering the arachnoidal trabeculse. The details of 

 these processes may be most easily studied in the region of the cerebral hemispheres; 

 in this situation the transformation of the tissues occurs at a later period than in the 

 basilar regions, for the differentiation of this mesenchyme follows the general plan 

 of development of the cartilaginous and bony cranium. 



The greatest problem in connection with the development of an external arach- 

 noid membrane naturally concerns the separation of this leptomeningeal tissue 

 from the pachymeninx. In the solution of this particular problem gross dissections 

 have been found of benefit. For this purpose, pig embryos of larger size were used, 

 and attempts were made to ascertain at what stage of development a true anatomi- 

 cal separation of the two membranes occurred. It was found that in embryo pigs 

 of about 40 mm. the dura over the calvarium could be well separated from the arach- 

 noid, but areas of unseparated tissue still persisted at this stage. This was also found 

 to be true in pig embryos of 50 mm. ; on the inner surface of the dura at this stage a 

 mesothelial cell pattern could be demonstrated, although areas of attachment to 

 the arachnoid existed. However, the differentiation of the periaxial mesenchyme 

 into the adult arachnoid does not occur coincidently with the possibility of a forceful 

 separation of the dura from the surface of the brain; but before this separation of 

 the pachymeninx can be made the mesechyme which will go to form the arachnoid 

 must undergo some differentiation. This process involves a condensation or accu- 

 mulation of mesenchymal elements directly in the secondary dural thickening; the 

 cells, with oval nuclei, soon form a continuous membrane of two or three cells in 

 thickness. Apparently soon after the cellular accumulation has been accomplished, 

 a separation of the dura from the arachnoid may be made. In certain areas, varying 



