THE DEVELOPMENT OF THE CRANIAL DURA MATER. 77 



tions persist in simple form until after the cerebro-spinal fluid begins to fill its extra- 

 ventricular bed; then, within a short time, the tissue becomes transformed by the 

 development within it of cartilage, so that in the human embryo the caudal half 

 of the chondro-cranium forms a ring of cartilage about the posterior portion of the 

 brain. On the inner side of this ring of cartilage the mesenchyme later shows a 

 marked condensation in the midst of the rarefied perimedullary tissues. In this 

 layer the nuclei soon become fewer in number and the cytoplasmic structures fibril- 

 lar, the whole resulting ultimately in the formation of the fibrous adult dura. The 

 mesenchymal condensations in the regions of the skull, where membranous bone for- 

 mation holds, go directly into a membrane of fibrous tissue, in the outer portions of 

 which bone is laid down. The details of these processes will now be taken up. 



In figures 30 and 32, photomicrographs from pig embryos of 13 and 14 mm., 

 respectively, the well-established vertebral differentiations and the now poorly 

 differentiated base of the skull are shown. From this stage upward the mesen- 

 chymal condensation in the head region proceeds rapidly. Thus at a stage of 

 17 mm. in the human embryo (fig. 60) the ventral portion of the vertebral canal 

 has become cartilaginous, while the base of the skull has also undergone the chon- 

 drogenous transformation in its more posterior portions. But of especial interest 

 in our problem is the line of mesenchymal condensation, which may now be traced 

 wholly around the brain-stem and hemispheres (fig. 60). The nature of this con- 

 densation is well shown in figure 61, an enlargement of the squared area of figure 60. 

 The mesenchymal nuclei have become closely packed and rather sharply differen- 

 tiated from the looser mesenchyme which in part goes to form the arachnoidea. 

 Figure 59 similarly shows this condensation proceeding upward to the vault. 



Examined in another plane, the process of mesenchymal condensation seems 

 to proceed much more rapidly in the posterior than in the anterior region. This is 

 brought out in a transverse section of a human embryo of 18 mm. (fig. 62). Here 

 the condensation is much more extreme about the medulla and roof of the fourth 

 ventricle than in the more anterior parts of the mesencephalon. The same general 

 appearance, typical of this stage, may be made out in figures 56 and 57 from a human 

 embryo of 14 mm.* (No. 144, Carnegie collection). In the slightly larger stages the 

 process of mesenchymal condensation about the nervous system becomes rapidly 

 more marked. This increase in the number of cells comprising the denser membrane 

 is shown in figures 64 and 65, photomicrographs of embryo No. 460 (21 mm.). 



The degree of condensation of the mesenchyme in the various stages of the 

 human embryo is followed quite closely in the pig embryo. The comparative 

 degree of differentiation coincides within a millimeter or two. Thus, in a section 

 from a pig embryo of 19 mm. (fig. 38), the degree of condensation about the roof of the 

 fourth ventricle is practically similar to that in human embryos of the same length. 



The phenomena just commented upon represent the stages concerned in the for- 

 mation merely of a cranial blastema and are related to the formation of the dura only 

 so far as it is out of this mesenchymal condensation that the periosteal portion of the 



*Measured on slide after sectioning. 



