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can be first distinguished, are composed solely of a semilransparent 

 and very delicate membrane. By degrees its density and thickness 

 encrease. It becomes firmer and more opaque ; and, about tlie mid- 

 dle of the second month after conception, it can with ease be shown 

 to consist of two layers. From the manner, in which the bone 

 is afterwards deposited, between the layers of this membrane, it 

 may probably be considered as the rudiments of the dura mater 

 and pericranium. These two membranes (for I shall now con- 

 sider each layer as a distinct one,) are connected by the medium 

 of a tissue, apparently cellular, and looser than the membranes 

 themselves ; but this medium does not resemble, nor do they, at 

 this period, contain between them any thing like cartilage. Their 

 nature, and the resemblance between them and other stmctures, 

 cannot, in consequence of their delicacy and imperfect organization, 

 be determined ; but, as well from some observations on them at this 

 period, as from their nature, when more fully developed, I suppose 

 them to be of the same structure as that tissue, which Bichat calls 



fibrous. 



About the end of the second month, the walls of the cranium 

 undergo a remarkable cliange. Cartilage is deposited between 

 the dura mater and pericranium in the base ; but not, as is sup- 

 posed, in the vault.* The deposition of the cartilage in the base 

 is even limited to particular parts. I have observed cartilage in 

 the situation, where the aethmoid, the body of the sphenoid, 

 the petrous portion and mastoid process of the temporal, and the 

 lower part of the occipital bones, are developed ; but, not in the 

 situation of the orbital processes of the frontal, 6r temporal 

 wings of the sphenoidal. 



• Kerkringius, Osteog. Faet. cap. II. 



