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



Quite similar pictures are obtained regarding the dura mater in the human 

 embryo. The relationships of the dura to the cisterna cerebello-medullaris are 

 shown in figure 94, a photomicrograph of a human fetus of 35 mm. (No. 199 in the 

 collection of the Carnegie Institution). In this reproduction the line of secondary 

 mesenchymal condensation (representing the outer membrane of the arachnoidea 

 and the inner surface of the dura) becomes widely separated from the occipitale 

 superius in its superior portion. 



In a fetal pig of 8 cm. the same general arrangements of the dura mater could 

 be made out. The inner surface of the dura was in places still fused with the outer 

 arachnoid membrane, but in other places the areas of attachment were lacking, so 

 that a true separation of arachnoid from dura had taken place. Along the peripheral 

 points of the tentorium the dura and arachnoid were still closely applied to each 

 other. The dura itself was of the same cellular, rather loose tissue, with a dense 

 inner surface. In places, as described in the younger stages, the dural tissue was 

 incorporated with the definitive perichondrium over certain cartilages or even over 

 parts of the same structure. In other places a definitive perichondrium may be 

 wholly lacking; in these areas the indefinite cartilaginous border gradually merges 

 into the dura. In still other situations an intermediate arrangement of dura and 

 perichondrium exists, where the cartilage is bounded by a somewhat condensed but 

 not fully developed perichondrium which is continuous with the dura. Everywhere 

 in the membranous sutures between the cranial cartilages or bones, the dura bridges 

 the gap as a loose, cellular tissue. Over the calvarium the dura appears solely as a 

 dense, rather fibrous membrane which is incorporated with and serves as the inner 

 periosteum. This dura over the hemispheres is continuous with the fibrous sutures 

 of the cranial vault. 



The findings in a fetal pig of 98 mm. were not dissimilar to those just recorded. 

 The dura was everywhere quite well developed, a rather loose cellular tissue except 

 over the hemispheres, where it showed a more fibrous character. In the region of 

 the occipito-atlantoid ligament the dura was fused with the ligamentous tissue, 

 while above (over the occipitale superius) the dura became a distinct, thick cellular 

 layer. The structure of the tentorium was wholly similar to the occipital dura. In 

 the basis cranii there are areas in which the dura is wholly fused with the periosteum 

 or perichondrium; in other areas it bridges the sutures or exists as a definite mem- 

 brane on the inner surface of a definite perichondrium. 



The dura mater in a fetal pig of 15 cm. did not vary greatly from those larger 

 stages already described. The tissue, however, had become somewhat more 

 fibrous. The prismatic attachment of the tentorium was no longer as large propor- 

 tionately, but the dura lining the occipitale superius remained a thick bulbous swell- 

 ing on the dorsal surface. But most striking of all the features in the specimen 

 was the very dense fusion of the dura of the calvarium with the fibrous sutures of 

 the cranium. No line of demarcation between dura and fibrous suture could be 

 made out; the two fibrous layers are anatomically one structure. 



