84 DEVELOPMENT OF CEREBKO-SPINAL SPACES IN PIG AND IN MAN. 



stellate or spindle-like nuclei, greatly exceeding in number the nuclei found in the 

 dense dura of the adult. Bone is being laid down in the outer portion of this dura 

 where it merges with the membranous skull. The lateral walls of the great sinus 

 are still free from fibrillar depositions. A somewhat analogous picture is afforded 

 by a photomicrograph of a specimen stained after Mallory's method, from a fetal 

 pig of the same length (fig. 105). In this specimen the outer portion of the dura, 

 incorporated into a part of the membranous skull, is quite dense with the fibrous 

 tissue; about the superior sinus, however, the decrease in the amount of fibrous 

 tissue is very striking. The falx is beginning to exhibit a fair degree of fibrillar 

 structure; it forms a definite division between the two hemispheres. 



In the larger fetuses, above 100 mm. in length, the process of formation in the 

 dura of denser and denser connective tissue proceeds rather slowly. It is realized, 

 however, that this fibrous transformation in fetuses of 10 cm. is very extensive, the 

 region about the sinuses alone remaining comparatively free from the development 

 of the fibrils. The chief difference between the dura of this stage and the dura of 

 the adult is a greater number of cell-nuclei in the fetal membrane. It is well, then, 

 to consider the cellular character of the fibrous membrane and the region about the 

 sinuses in the larger stages. 



In a human fetus of 125 mm. (No. 900-H) the dura is quite fibrous, but still 

 contains an increased number of the stellate and spindle forms of nuclei; likewise, 

 about the superior sinus the tissue is an immature myxomatous structure, fairly free 

 from connective-tissue fibrils. This increased number of nuclei in the dural tissue 

 holds also for human fetuses of 165 mm. (as in No. 745), but seems slightly decreased 

 as compared with the smaller specimens. The lateral wall of the great sagittal 

 sinus in this stage possesses distinct bands of white fibrils, but the tissue is much 

 looser and more cellular than the fibrous dura over the hemispheres. These phe- 

 nomena may be made out in similar stages of the fetal pig, as shown in figure 106, 

 a photomicrograph from a specimen of 17 cm. In this specimen, treated by 

 Mallory's stain, the superior longitudinal sinus is shown surrounded by a clear zone 

 in which the deeply staining fibrils are comparatively few in number. On each 

 lateral wall of the venous channels distinct fibrous bands may be made out, lying 

 in the looser, more immature tissue. The lower portion of the falx has assumed 

 quite an adult character. 



Gradually the conversion of the tissue about the cerebral sinuses into the adult 

 structure progresses. Thus, in both human and pig fetuses of 20 cm. length, the 

 dura mater has acquired practically all of its adult features. Everywhere over the 

 cerebral cortex the dura is characterized by dense layers of interlacing strands of 

 white fibrous tissue, but the number of nuclei in these bundles may still be slightly 

 greater than in the adult structure. In the more posterior regions, at this stage of 

 20 cm., the lateral walls of the sinus sagittalis superior are found to be completely 

 occupied by the white fibrous tissue; in the anterior portion of the sinus much 

 thinner tissue, resembling myxomatous structure, appears, as shown in figure 107. 

 But in this specimen the invasion of the area about the great venous channel by 



