SECT. I06.] 



OSSEOUS SYSTEM. 



203 



histology of them. Nevertheless, there is by no means a general coincidence 

 of opinion on the subject. With reference to the histology, I must refer the 

 reader to the remarks made in my Micr. Aiiat. ; with regard to the morpho- 

 logical aspect of the question, I will only remark, that the doctrine of the 

 primordial cranium and the secondary bones is in a great measure indepen- 

 dent of the question, whether the latter arise from cartilage or connective 

 tissue. This is supported by the fact, that some bones proceed directly from 

 the cartilaginous primordial cranium, whilst the others arise externally upon 

 these, and are not pre-formed. 



Fig. 88. 



§ 106. All the cranial bones not primarily cartilaginous, first 

 appear in the form of a very limited, elongated, or rounded bone- 

 nucleus, consisting of a small quantity of fundamental substance, 

 and some few lacunae, and surrounded by a small amount of a 

 soft blastema. The manner in which this nucleus arises has not 

 hitherto been observed ; still, from the method in which it is 

 further developed, it may be deduced with certainty, that a short 

 time previous to its appearance, a small lamella of the above- 

 described soft blastema is formed in its place, which then, by the 

 deposition of salts and the metamorphoses of its cells, ossifies 

 from one point outwards. If an ossifying nucleus, for instance in 

 the parietal bone, be once commenced, it advances in such a 

 manner that, whilst the mem- 

 branous blastema grows hori- 

 zontally, a delicate layer of 

 reticulating osseous trabecular 

 soon arises, which sends out 

 fine rays into the still unossi- 

 fied blastema (fig. 88). If it 

 be examined more narrowly, 

 it is found that the individual 

 spicules of bone in the mem- 

 branous blastema have arisen 

 by the ossification of its ele- 

 ments, and have, as it were, con- 

 sumed it in the places which 

 they occupy, remains of it be- 

 ing left in their interstices ; 

 further, that the formation of 

 the osseous elements takes 

 place entirely in the same man- 

 ner as in the sub-periosteal 

 deposits, the rays of bone when traced to their growing-points 



Parietal bone of a foetus 14 weeks old. Magnified 

 18 times. 



