DEVELOPMENT OF THE SECONDAKY BONES. 359 



the upper half of the expanded portion of the occipital bone, the 

 parietal and frontal bones, the squamous portion and tympanic ring 

 of the temporal, all the bones of the face, except the inferior tur- 

 binated bones ; and apparently the internal lamella of the pterygoid 

 process of the sphenoid bone. It will, however, appear that these 

 bones are developed in precisely the same manner as the periosteal 

 layers, or compact tissue of the other bones. 



The secondary bones of the cranium all commence in a mem- 

 braniform blastema lying between the dura mater and the integu- 

 ments, whose growth advances with the development of the osseous 

 tissue within it. The latter commencing in a single point, radiates 

 in all directions and thus forms a delicate lamina of reticulated 

 osseous spiculae, giving off slender rays into the still unossified 

 blastema. Minute examination shows that the spiculae are formed 

 by the ossification of the elements of the blastema, though to a 

 certain extent the latter is absorbed to give place to them, while it 



still fills the interstices between them : and that the formation of the 



7 | 



bone-tissue proceeds exactly in the same way as in the periosteal 

 layer of the primary bones. (Kolliker.) At first, the growth pro- 

 ceeds in a superficial plane only, the rays forming a network as they 

 come into contact with each other, as shown in Fig. 229. Addi- 

 tional layers are, however, soon added to both surfaces of the ori- 

 ginal one, and thus the structure becomes thicker, and at the same 

 time also more compact. The thickening layers are, however, re- 

 ferable to the periosteum which is found on the secondary bones 

 soon after their development has begun ; so that in fact only the 

 primary layer presents any apparent peculiarity in development. 

 The bone increases in extent by the formation of new blastema in 

 contact with that just about to be ossified, until it has attained to 

 its full size; and it is constantly increasing in thickness by the ad- 

 dition of the periosteal layers as just explained. Interstitial changes 

 are also at the same time going on, and the final result is, the for- 

 mation of the bones with their compact layers and Haversian canals; 

 and their cancelli internally, constituting, in case of the cranial 

 bones, the diploe. The cells in the blastema never resemble carti- 

 lage-cells, except those at the edges of the newly formed bone. But 

 Kolliker doubts if even these be true cartilage-cells. It has already 

 been seen that it is not true cartilage which connects the cranial 

 bones together in the adult, but collagenous tissue instead (p. 347). 

 There is usually but one centre of ossification for each of the se- 



