GENERAL HISTOLOGICAL DIFFERENTIATION OF CEREBRO-SPINAL SPACES. 63 



VII. GENERAL HISTOLOGICAL DIFFERENTIATION OF THE CEREBRO-SPINAL 



SPACES. 



The general problems concerned in the formation of the meninges and of the 

 spaces inclosed within them deal with the gradual adaptation of a primitive undiffer- 

 entiated mesenchyme to the anatomical and physiological requirements of the adult. 

 Originally the meninges were held to be derived from the same epidermal infolding 

 which gave origin to the central nervous system; then, with increasing knowledge of 

 the structure, the dura alone was said to be a product of the middle germ-layer; 

 and finally, by the researches of His^ 25 ) and of Kolliker/ 31 ) the mesenchymal origin of 

 the three meninges was established. The general process of the differentiation and 

 the stages in this transformation have not been reported in great detail; here, too, 

 the investigations must have an outlook for physiological anatomy as well as for 

 pure morphology. 



It may be well to comment briefly on the relationships of the three meninges 

 found in adult mammals. The dura is well established as the fibrous-tissue envelope 

 of the leptomeninges and the central nervous system. But there is a tendency 

 to regard the arachnoid and pia mater as constituting one structure the lepto- 

 meninges or "pia-arachnoid," in the terminology of Middlemass and Robertson^ 39 ). 

 This difference of opinion in regard to the two inner meninges is due to their struc- 

 tural and intimate relationships. The arachnoid may well be assumed to be a 

 single membrane, worthy of being regarded as a single structure if one considers only 

 its outer continuous membrane as the essential structure. But the inner surface 

 of this membrane sends processes inward to fuse with the pia mater, which is so 

 closely applied to the nervous tissue. These processes divide the subarachnoid 

 space (included between arachnoid and pia) into the well-known meshes in which 

 the cerebro-spinal fluid circulates. From the standpoint of these channels (the 

 subarachnoid spaces) the arachnoid constitutes the parietal and the pia the visceral 

 layer. Thus the intimate structural unity of the two membranes seems, in the opinion 

 of many investigators, to warrant their designation as a single membrane. This 

 view, however, has been strongly opposed by Poirier and Charpy( 45 ), who considered 

 the distinction of three meninges very essential. Hence, in considering the trans- 

 formation of tissues in the embryo, regard must be had for the dura as a well-differ- 

 entiated structure, and for the leptomeninges as units, but certainly to be regarded 

 from the standpoint of the subarachnoid spaces. In this connection SterziV 53 ) 

 observations on the comparative anatomy of the meninges are of interest. It will 

 be recalled that the dura in lower forms becomes well established before the lepto- 

 meninges emerge from a primitive mesenchyme. 



THE PERIAXIAL MESENCHYME. 



Surrounding the central nervous system in young embryos is a rather thick 

 cushion of undirTerentiated mesenchyme, similar in all respects to the undifferen- 

 tiated tissue in other parts of the embryo. But very soon in the course of develop- 

 ment the nuclei in this mesenchyme increase along the clear marginal zone of the 



