82 DEVELOPMENT OF CEEEBRO-SPINAL SPACES IN PIG AND IN MAN. 



from this layer. The findings, however, in this investigation, are against any addi- 

 tion of perichondrium to the dural tissue; histologically, a definitive perichondrium 

 is a membrane entirely apart from the dural condensation. 



Over the cerebral hemispheres the dura of the cranial vault offers more diffi- 

 culties of study than does that of the basilar regions. With the formation of a 

 blastemal condensation over the whole vertex an extension of the dorsal membrane 

 to form the membranous skull there occurs very quickly a condensation to form 

 the dura. This condensation may be first detected as a continuation anteriorly 

 of the leaflet of the tentorium cerebelli, which stretches forward from the prismatic 

 zone of the tentorial attachment. This zone of condensation is wholly similar to 

 the narrow line of the mesenchymal thickening which was found in the more basilar 

 regions of the skull. This zone of condensation occurs just within the cranial 

 blastema and may be traced upward over the mesencephalon and laterally around 

 the rapidly enlarging hemispheres. As the distance from the cerebellar attachment 

 increases, the zone tends to approach the blastema, except in those regions in which 

 the precursors of the dural veins occur. In such a situation this inner dural zone 

 swings inward to encompass the vessels. Between this inner line of the dura (repre- 

 senting also the outer surface of the arachnoid) the same rather loose cellular tissue 

 exists. 



From the falx cerebri a zone of dural condensation in the mesenchyme spreads 

 laterally also; this gradually may be traced anteriorly and laterally until fusion 

 with the similar lines of condensation from the basis cranii and the prismatic zone 

 of the tentorium are reached. The condensation connected with the falx cerebri, 

 however, is not an extensive process, the greater part of the hemispheres being 

 covered by the development from the basis cranii and from the tentorium. It must 

 be understood, however, that there is no active migration of this line of condensa- 

 tion, for the whole process is a development in situ. The appearance of an active 

 extension is derived solely from the study of various stages and the increased area 

 of condensation appears as an increment which has developed at the terminal points 

 of the previous condensation. 



The amount of dural tissue delimited in the mesenchyme by the secondary 

 zone of condensation is not great in the region of the vertex. It is a thin layer which 

 fuses to the inner surface of the cranial blastema. At the stage of this fusion the 

 blastema has become somewhat fibrous and it constitutes the membranous skull. 

 In this fibrous tissue (the union of the blastema and the dura) bone is deposited, but 

 only in the outer layers. The phenomenon is easily studied in any suitable stage, 

 for the sutures between the flat cranial bones remain incorporated with the inner 

 membrane the dura which includes the periosteum. Hence, over the cranial 

 vault, the dura and periosteum become incorporated as a single membrane; this 

 serves as the membranous skull, into the outer layer of which bone is deposited. 



In the basis cranii, as soon as ossification of the cartilaginous skull takes place, 

 the dura becomes incorporated as the periosteum in a manner similar to that which 

 takes place in the cranial vault. While no definite relationship of dura to the peri- 



