434 THE PREFUNCTIONAL AS CONTRASTED WITH 



The fine architecture of bones appears also to be determined 

 in the same way. Here, too, structures which are mechanically 

 adapted in great detail to their functions are not determined heredit- 

 arily. On the other hand, the general form of bones is predeter- 

 mined in great detail by chemo-differentiation. Certain depressions 

 in the surface of avian bones appear to result from mechanical in- 

 teraction with neighbouring bones, but all the projections from the 

 surface, including the joint-structures, will arise in isolated bones 

 grown in culture media (see p. 225). It may prove that the 

 cartilaginous rudiment is rigidly predetermined, whereas the bony 

 structure, being secondary from the start, is always dependent 

 in its differentiation. 



The coarse structure of a bone is, then, a result of chemo-differ- 

 entiation during the prefunctional period, but function is necessary 

 for the perfection of its finer structure, viz. the orientation of its 

 spicules. Function is also necessary for the normal growth of 

 bones. If one leg of a new-born animal is kept immobile and non- 

 functional, the long bones remain much slenderer than in the used 

 limb of the other side. On the other hand, if a leg is subjected to 

 changed function, as in the case of the hind legs of puppies born 

 without front legs, the hind legs, from the practice of hopping, 

 assume the proportions characteristic of hopping animals such as 

 the kangaroo." 



With regard to the blood-system, little is known as to how much 

 of the broad lines of its architecture may be determined by chemo- 

 differentiation. What is certain, however, is that a very great deal 

 of its detailed architecture, as regards the size of vessels, the angles of 

 their branchings, and the courses which they follow, are determined 

 hydrodynamically. The pressure of the blood moulds the vessels in 

 such a way as to offer the least resistance to its flow.^ 



Lastly, instances may be given of functional changes involving 

 cell-form as well as the total size of an organ and the development 

 of its parts. The first case, like so much of the functional differ- 

 entiation of the blood-vessels, shows the effect of pressure of a 

 contained fluid on the walls of its container. The urinary bladder 

 of a dog of medium size normally evacuates a quarter of a litre of 

 fluid per day. The wall of the bladder is composed of smooth 

 muscle cells and is about half a millimetre thick. By means of a 



'*■ Fuld, igoi. - Oppel and Roux, 1910. 



