162 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



comparative anatomy. Only the base of the human skull is 

 developed in cartilage, the rest is developed in membrane. How 

 has that come to be ? The brain of amphioxus, if it can be said 

 to possess one, is wrapped in a membranous covering. In fishes 

 with cartilaginous skeletons this embryonic mesoblastic capsule 

 becomes chondrified — plates of cartilage develop in it. As in the 

 spinal column, the process of chondrification begins at the base 

 and spreads slowly round to the crown or dorsum of the head. 

 The cartilaginous cranium is an advance on the membranous 

 stage. In many fishes a further most important element is 

 added. The dermal bony plates, to which the placoid scales are 

 fixed, are applied to the cartilage over the sides and dorsum 

 of the skull. Thus to the cartilaginous element of the skull 

 is added a third element — bone formed in membrane. Now in 

 the mammalian skull, and especially in that of man, the cerebral 

 vesicles grow so quickly that long before the process of chondrifi- 

 cation has had time to spread in the membranous capsule from 

 the base to the crown, the dermal bones have formed, and thus 

 supplant the cartilage on the calvarium. Hence in the human 

 skull, while the process of chondrification occurs in the base, and 

 afterwards undergoes ossification, the whole calvarium and sides 

 of the skull are formed by bones which, historically, are dermal 

 bones, and hence are formed directly in membrane. The dermal 

 bones of the human skull are: (1) the frontal, (2) the parietal, 

 (3) the inter-parietal part of the occipital (the part above the 

 superior curved lines), (4) the squamous part of the temporal. 



Thus the calvarial part of the skull passes directly from the 

 membranous to the bony stage, while the base of the skull, like 

 the spinal column, passes through three stages : (1) membranous, 

 (2) cartilaginous, (3) bony. It will be thus seen that the base 

 of the skull, developed in cartilage, is the most ancient part, 

 while the dermal bones, which form the calvarium, represent a 

 later addition. 



Development of the Roof (membranous or dermal part) 

 of the Skull. — In the 7th week of foetal life there appear on 

 each side of the membranous cranial capsule four centres of 

 ossification : 



(1) For the frontal bone at a point which becomes afterwards 

 the frontal eminence (Fig. 130); 



