THE DEVELOPMENT OF THE CRANIAL DURA MATER. 75 



the cerebro-spinal fluid would, in its course from the ventricle to the subarachnoid 

 space, form a subpial extravasation. It would seem that this modification of the 

 pia is designed to meet the particular need and function of this region. 



THE ADHESION OF THE PIA MATER TO THE CEREBRAL TISSUE. 



It is a well-known fact in embryology that the pia mater and the periaxial 

 mesenchyme in poorly dehydrated specimens split away from nervous tissue, but in 

 adult preparations, if the meninges and brain are dehydrated in a block, the sepa- 

 ration of the tissues occurs between the dura and the arachnoid, or (in more excep- 

 tional instances) the dura and arachnoid come away, leaving the pial layer closely 

 applied to the cortical tissue. It is quite difficult in any adult mammal to separate 

 the pia from the brain tissue. Realization of these peculiarities in the degree of 

 adhesion of the pia led to an attempt to ascertain what structures were involved in 

 the attachment of this mesodermal layer to the epidermal nervous system. The 

 results of this attempt add nothing to the ultimate solution of the problem, but 

 are perhaps of sufficient interest to justify brief presentation. 



Two theories in explanation of this adhesion of the pia immediately suggested 

 themselves. One of these concerned a possible growth into the pia of neuroglial 

 elements, causing an intimate association between the pia and the cerebral cortex. 

 Our findings in reference to the neuroglial outgrowth in fetal pigs gave no reliable 

 basis for the assumption. The second theory dealt with a diminution in the elas- 

 ticity of the walls of the perforating blood-vessels which supply the nervous system. 

 The early embryonic vessels, with walls composed solely of endothelium, when sub- 

 jected to the distortions of poor dehydration, might possibly offer less resistance to 

 the separation, so that the pia would come away from the nervous tissue. In the 

 later stages, however, the thicker-walled perforating vessels would naturally oppose 

 such a cleavage, so that the pia would remain adhering to the cortical tissue. This 

 theory is also purely an hypothesis, although it does not seem unlikely, especially 

 if one takes into account a possible connection of the pia with the perivascular 

 system. In examining blocks of the meninges and brain tissue taken together it 

 was found that the pia mater separated cleanly from the nervous tissue in fetal pigs 

 15 cm. in length. Beyond this stage the arachnoid might remain in adhesion to the 

 dura, but in such cases there was always found a layer of cells on the outer side of 

 the cortical tissue, constituting a true pia mater. 



IX. THE DEVELOPMENT OF THE CRANIAL DURA MATER. 



The dura mater, like the two other meninges, is derived from the mesenchyme 

 about the central nervous system. The researches of Sterzi (53) on the comparative 

 anatomy of the meninges furnish additional evidence for this conception in the 

 higher mammals. The origin of the pachymeninx from the middle germ-layer is 

 now well established. But there is lacking in the literature a comprehensive 

 account of the formation of this fibrous envelope. The gross generalities of the 

 process are given in part, but there is an almost total absence of the more intimate 



