THE DEVELOPMENT OF THE CRANIAL DURA MATER. 79 



line of condensation about the whole posterior chamber may be made out. This 

 lies within the area of the cartilaginous skull and bounds the subarachnoid spaces. 



This same process of formation of dura holds for the formation of the basilar 

 dura in the more anterior portions of the cranium. The appearance of the secondary 

 zone, narrow and rather dense, may be made out inclosing the more cellular mesen- 

 chyme which extends to the cartilaginous skull. The same process also endures 

 for the formation of the dura of the calvarium, but here the addition of tissue from 

 the undifferentiated mesenchyme is undoubtedly very small in amount. This will 

 be discussed in a later paragraph. The various stages in the formation of this 

 secondary condensation which goes to form the major portion of the dura may be 

 fairly well studied in any one embryo of suitable stage, because the process, as 

 pointed out above, begins in the basilar portion of the cranium and extends upward. 

 Likewise, the condensations directly beneath the region of the dorsal membrane 

 are delayed as compared to those of the lateral regions. 



Some of the phenomena shown in the formation of the dura mater are illustrated 

 in figures 46, 76, 77, 78, 79, and 94. Throughout these figures the letters dmc 

 refer to the line of the secondary mesenchymal condensation, which borders inter- 

 nally the dura and which goes to form the outer membrane of the arachnoidea. 



In figure 46, a photomicrograph of a pig embryo of 32 mm., the dura mater 

 (dmc) is shown as a somewhat condensed tissue separated a slight distance from the 

 chondro-cranium. On the basilar surface, the inner line of dural tissue is quite 

 remote from the inner surface of the basioccipital. Tracing this line of condensa- 

 tion forward, it is soon seen to merge more closely with the perichondrium* of the 

 basioccipital. More anteriorly it again leaves the occipital plate and after a brief 

 interval it fuses with the temporal perichondrium. Continuing slightly more 

 anteriorly the dural process toward the mesencephalic angle may be made out; this 

 appears as a doubled membrane at its basal attachment. In its further prolongations 

 the dural surface is at times a distinct structure; at other times it is completely 

 fused with the perichondrium. 



Posteriorly, in this figure 46, the line of dural condensation (incorporated also 

 with the outer arachnoidea) may be traced upward around the cisterna cerebello- 

 medullaris. This line of condensation is seen to lose its definitive character as it 

 curves inward toward the chorioid plexus of the fourth ventricle a phenomenon 

 shown particularly well in figures 77 and 79, taken from the same pig embryo of 32 

 mm. The dura in this termination may be said to be in its formative stage; but 

 dorsally, over the mesencephalon, the inner surface of the dura again becomes a 

 definite membrane, as shown in figures 76 and 78. In the latter figure it is shown 

 inclosing a wide mesh of dural vessels, between the arachnoidal surface and the 

 membranous skull. Anteriorly, again, it seems to lose its definite line of con- 

 densation. 



*The term "perichondrium" is used throughout this paper to designate only the very dense cellular line delimiting 

 the edge of the cartilage. This dense zone is composed of the nuclei of the cartilage, crowded together, and represents 

 probably some phenomenon of the growth or resorption of the cartilage. In a much broader sense, the whole dural tissue, 

 lying between the line of secondary condensation and the cartilaginous border, could be termed "perichondrium, " as it 

 probably represents the sole internal membrane which could be stripped from the cartilage. 



