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



pachymeninx may be derived. The degree of condensation referred to in the figures 

 has been solely of the blastemal type, but in some of the specimens this simple conden- 

 sation is seen only in the more cephalic portions of the cranium. Thus, in the figures 

 (64 and 65) taken from embryo 460, the mesenchymal condensation is still of the 

 simple undifferentiated type, whereas in this same embryo the more caudal sections 

 show a chondro-cranium which is well developed. The process of formation of the 

 cranial dura, then, is one which begins in the basilar portions of the cranium and 

 proceeds from these points into the region of the calvarium. In general, all of the 

 phases of this transformation into dura may be found in one specimen of sufficient 

 and suitable size, the basilar differentiation representing the advanced stages, while 

 the steps in the differentiation are found in the areas nearer the vertex. 



It is quite difficult to decide exactly what importance the primary condensation 

 of mesenchyme maintains in the formation of the dura, because, coincident with the 

 chondrification of the blastema, there occurs another condensation which forms the 

 line of division between the inner surface of the dura mater and the outer arachnoid 

 membrane. The first evidence of this secondary perimedullary condensation is 

 found in pig embryos of about 17 mm. In these specimens, in the narrow space 

 formed by the mesencephalic flexure, mesenchymal cells collect together in the form 

 of a fairly definite membrane. After its primary beginning in this area, the narrow 

 line of its thickening may be traced to the basis cranii in embryos a little larger. 

 In slightly older stages this secondary line of condensation is found to be fairly exten- 

 sive throughout the area between the middle and posterior cranial chambers. 



At a stage of 20 to 21 mm. the whole basilar portion of the cranium shows 

 evidence of this secondary line of condensation lying between the pia mater and the 

 cartilaginous skull. The condensation occurs in the outer portions of the loose tissue 

 which, as shown in a foregoing section (No. vn) becomes the subarachnoid trabec- 

 ulse. The line of condensation is not broad on section; it comprises a cell-layer 

 from three to six cells in thickness. Between this cellular border and the cartilagi- 

 nous skull the tissue rapidly differentiates (a process seemingly synchronous with 

 the development of this membrane). This tissue, which maintains dural relation- 

 ships, is far more cellular and compact than the original perimedullary mesenchyme. 

 Even without the rather dense line of division in the mesenchymal tissue, the dural 

 structure can be easily outlined by its characteristic appearance. 



The original dural condensation between the two wings of nervous tissue which 

 unite in the mesencephalic flexure can be traced in slightly later stages around into 

 the tentorium cerebelli. This structure develops as a wholly similar mesenchymal 

 thickening in the midst of the perimedullary mesenchyme. The tentorium consists in 

 these embryos of 20 to 25 mm. of two thin lateral plates which widen at their cranial 

 attachments into prismatic areas. These areas, which finally lodge, in the two layers 

 of dura, the sinus transversus, are characterized by the same dense type of mesen- 

 chyme. The peripheral edges of the prismatic portion of the tentorium spreads 

 caudalwards as a definite line of condensation. In the earlier stages this line 

 becomes indefinite as it extends from its tentorial attachment, but finally a similar 



