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



details of the transformation. One of the most essential points in the process con- 

 cerns the relationship of the dura to the bony coverings of the cerebro-spinal axis. 

 Does the adult dura serve as the periosteum of the bony skull? In the standard 

 text-books of anatomy the adult human dura is described as being composed of 

 two layers. In the skull these layers split, to comprise the walls of the great venous 

 sinuses. The outer layer of the dura serves as the periosteum for the bony skull, 

 but below the foramen magnum the two layers separate to inclose the epidural space. 

 The outer dural layer in this spinal region adheres to the inner surface of the bony 

 vertebral column, where it functions as the periosteum; the inner layer here becomes 

 the spinal dura. 



In this account of the adult dura mater there is indicated a very suggestive 

 periosteal relationship which implies an embryological basis for the disposition of 

 the two layers of the membrane. It must be granted, however, that this division 

 of the cranial pachymeninx into two layers is quite arbitrary; there is nothing in 

 the general histology of the fibrous covering to suggest such a halving except its 

 division about the sinuses and its spinal relationships. 



THE GENERAL PROCESS OF THE FORMATION OF CRANIAL DURA. 



The first evidence of the development of the pachymeninx is found in the 

 basilar region of the skull, where the mesenchyme thickens, to form eventually the 

 bony covering of the brain. This thickening of the mesenchymal elements results 

 not only in the formation of the chondro-cranium, but also in the final formation of 

 the bony skull and possibly its internal periosteum and dura. In the process of 

 differentiation the condensation of mesenchyme in the early stages gives no index 

 of the varied character of the resultant tissues, so that, in the first place, the study 

 of the process was necessarily related to the more adult stages. In this paper, how- 

 ever, the whole history of the dura will be detailed chronologically, beginning with 

 the earliest stages. 



Bardeen^ 2 ) has given data on the first appearance of the mesenchymal conden- 

 sations which go to form the blastemal phenomena in both the cranial and spinal 

 regions. The blastemal vertebrae become fairly well differentiated in human embryos 

 during the first month of intra-uterine growth. At the end of the first month, in 

 the occipital region, three fairly well-marked occipital myotomes may be made out; 

 these afterwards disappear. "During the earby part of the second month the mem- 

 branous anlage of the skull becomes extensively developed. The roof of the cranial 

 cavity is formed by a dense membranous layer, which first becomes marked at the 

 side of the head in embryos from 9 to 11 millimeters in length" (Bardeen). 



These evidences of a primary mesenchymal condensation about the central 

 nervous system are concerned in the problem of the differentiation of the dura only 

 in so far as they indicate the onset of the process which will give rise to the bone and 

 possibly the periosteum a part of the dura about the cerebro-spinal axis. Gaupp( 19 ) 

 has already pointed out that this cranial blastemal condensation gives rise to these 

 adjacent but wholly different structures. These cranial mesenchymal condensa- 



