376 THE MECHANICAL FUNCTIONS. 



portunity of observing the earliest stages of its 

 deposition, and of accurately following the sub- 

 sequent changes it undergoes. 



The parts of the embryo animal, which are 

 destined to become bone, partake of the soft 

 and gelatinous consistence, which, at that early 

 period, characterises all the textures of the 

 body; and they can hardly, indeed, be dis- 

 tinguished from the semi-fluid portions which 

 surround them. In process of time, when the 

 vascular circulation of the blood has been estab- 

 lished, and the newly formed arteries have ex- 

 tended their branches over every part of the 

 nascent organization, those vessels which are 

 appropriated to the task of forming the bones, 

 arrive at the pulpy masses where their work is 

 to commence. As sculptors, before working upon 

 the marble, first execute a model of a coarser and 

 more plastic material, so the first business of 

 these arteries is to prepare a model of the future 

 bone, constructed, not with the same material 

 of which it is afterwards to consist, but with ano- 

 ther of a simpler and softer nature, namely car- 

 tilage. In every case, then, cartilage is first 

 formed, and becomes visible by its greater opacity 

 when compared with the adjacent jelly. It is an 

 exact representation, in miniature, of the bone, 

 which is, in due course, to take its place. It is 

 evident that until the other parts of the fabric 

 have proceeded so far in their developement as 



