360 



THE TISSUES. 



condary cranial bones; or for each lialf, when one is symmetrical. 

 The spaces left between them at birth, from the fact that their angles 



Fig. 229. 



Process of ossification in parietal bone of an embryo-sheep of two and a half inches in length. 

 The small upper figure represents the bone of the natural size. The larger figure is magnified about 

 12 diameters. The curved line (a, 6) marks the height to which the subjacent cartilaginous lamella 

 extended. A few insulated particles of bone are seen near the circumference, an appearance which 

 is quite common at this stage. 



are still undeveloped, are termed the fontanelles ; and this condition 

 allows an overlapping of the bones of the vault of the cranium, by 

 which parturition is very much facilitated. 



The secondary bones are more vascular while growing than after- 

 wards ; more so even than the periosteal layer of the other bones ; 

 many of the vascular canals afterwards becoming much contracted, 

 or even obliterated. 



For the facts in regard to the precise period when each bone is 

 developed in the foetus, the works on anatomy are referred to ; the 

 law being that the bones, and even the parts of bones, first needed in the 

 skeleton are first developed. 



Remarks. 1. In regard to the amount of osseous tissue developed 

 in a given time in a young animal, the following facts may be 



