THE DEVELOPMENT OF THE CRANIAL DURA MATER. 81 



The falx cerebri forms in the pig and human embryo by a process similar to 

 that of the inner portion of the dura mater. In the sulcus between the two cerebral 

 hemispheres the mesenchyme remains undifferentiated until quite late; then there 

 appears in the posterior portion a narrow zone of condensation which soon presents 

 two lateral surfaces separated by a layer of rather loose cellular tissue, similar in 

 all regards to the dural tissue already described. This zone of condensation spreads 

 forward to comprise the whole falx. The double surfaces of this membrane finally 

 separate into two parts, forming the outer surface of the arachnoidea and the inner 

 surface of the falx. At the cranial attachment of the falx, the loose tissue forms 

 a prismatic base, containing the superior sagittal sinus and spreading laterally over 

 the denser dura of the calvarium. The whole appearance of this region, which will 

 again be referred to, is that the falx has been added onto the dura of the vertex. 

 Its time of initial appearance is later than that of the rest of the cranial dura and 

 there is apparently no additional acceleration of development. Hence the dural 

 tissue in the falx cerebri presents, in appropriate stages, a more immature type of 

 differentiation than does the adjoining dura. 



The process of the formation of the dura is not wholly a simple one due to the 

 relation of the adult dura to, or its function as, the inner periosteum of the skull. 

 In the figures already referred to, the almost complete fusion in some areas of the 

 inner line of dural condensation with the perichondrium has been commented upon. 

 In other situations definite separations of the inner dural surface from the peri- 

 chondrium occurred; in still other regions no perichondrium could be made out as 

 a definite membrane. These differences in relationships of the dural tissue to the 

 line of the perichondrium can not at present be wholly explained, but some indica- 

 tion of the meaning of the process can be given. 



Out of the original cranial blastema, as described by Gaupp< 19 ), there develops 

 the cartilaginous and bony skull, the periosteum, and the dura. But the observa- 

 tions recorded above indicate that by far the major portion of the dura is formed by 

 a secondary mesenchymal condensation, which was indicated by a thin zone of 

 more condensed cells on its inner border. This inner zone ultimately separated to 

 form the inner surface of the dura and the outer membrane of the arachnoidea. The 

 tissue included between this inner line of condensation and the cranial wall gradually 

 differentiated into a more condensed but still a loose cellular tissue and finally 

 became a fibrous dura. 



In all cases the dural tissue extends from the inner line of condensation to the 

 cranial blastema, to the perichondrium, or to the cartilage of the skull. The pres- 

 ence of a definitive perichondrium can not at present be explained, but apparently 

 the perichondrium is manufactured by the cells of the original cranial blastema and 

 not by the dural tissue which lies in approximation to it. When a definite peri- 

 chondrium is found, it seems quite uninfluenced by the dura; at other times a fusion 

 of an indefinite perichondrium with the dura seems to occur. The fusion of the 

 perichondrium with the dural tissue derived from the secondary mesenchymal con- 

 densation may occur, so that the small outer portion of the dura may be derived 



