THE DEVELOPMENT OF THE CRANIAL DURA MATER. 83 



chondrium could be made out in the earlier stages, the later function of the dura as 

 the inner cranial periosteum is quite obvious. Thus the adult relationships of the 

 dura are obtained. But it is quite difficult to decide to what extent the dura (or 

 internal cranial periosteum) is derived from the primary cranial blastema. It 

 seems probable that this blastemal condensation, in its final resolution into bone, 

 may contribute, in the form of a periosteal element, somewhat to the formation of 

 the dura. Such an addition is very difficult of verification; certainly the greater 

 part of the dura is derived by the secondary condensation from the perimedullary 

 mesenchyme. 



Before giving details of the fibrosis of the dura, it may perhaps be interesting 

 to point out a peculiarity of the primary cranial blastema, which does not seem to 

 be connected directly with the formation of the dura. This concerns the tendency 

 of the membranous skull to form more than one layer in its original zone of conden- 

 sation. In certain areas, as in figure 64, from a human embryo of 21 mm., the dorsal 

 membrane is shown split into two layers. Somewhat similar to this is the occur- 

 rence of two zones in the cranial blastema of a pig embryo of 23 mm. (figs. 22 and 

 101) . In this latter figure a less cellular outer layer and a more cellular inner layer are 

 seen. Neither of these have particular significance in the formation of the meninges, 

 although the inner layer in early stages actively functions as a fluid retainer. 



The question of the development of fibrous tissue in the dura mater in the 

 course of its development requires consideration here. This phase of the problem 

 concerning the formation of the pachymeninx has been followed, in this study, in 

 the dura of the vertex about the sinus sagittalis superior. The tissue was removed 

 in blocks, including the meninges and cortex cerebri, and was then sectioned in the 

 coronal plane. For the most part the deposition of fibrous tissue was studied in 

 sections stained with hematoxylin and eosin; the findings were controlled by treat- 

 ing other sections from the same blocks with Mallory's connective- tissue stain. 

 In this way the general histogenesis of the dural tissue could be well investigated. 



Sections from such a block from a human fetus of 76 mm. (No. 1134, Carnegie 

 collection) showed the dura to be composed of fibrous tissue everywhere except in 

 the region of the great sagittal sinus. About this sinus an immature, almost embry- 

 onic, type of loose myxomatous tissue was observed. The fibrous tissue comprising 

 the dura elsewhere is of a quite cellular, somewhat immature type of white connec- 

 tive tissue, with a considerable number of true fibrils. A wholly similar picture is 

 found in a section, stained by Mallory's method, of a block from a fetal pig of 80 mm. 

 (fig. 104). Unfortunately the cellular character of the fibrous dura is not brought 

 out, but the photomicrograph shows well the avoidance of the lateral walls of the 

 sinus by the process of fibrosis. The more embryonic type of tissue in the region 

 between the hemispheres is also well presented. 



The dura mater of a human fetus of 100 mm. (No. 928-E, Carnegie collection) 

 possesses fewer nuclei in a given area than does the dura from the specimen of 76 mm. 

 (No. 1134). The tissue is fibrous, except in the immediate region of the sinus 

 sagittalis superior; but interspersed among the connective- tissue fibrils are many 



