Chap, in.] OF GROWTH IN THE ANIMAL KINGDOM. 287 



metamorphosed into another ; never, for example, does a mus- 

 cular fibre become a nervous fibre, &c.^ 



The nucleus is usually the centre of genesis and of evolution of 

 the anatomical element. It is the nucleus which habitually 

 appeai-s the first ; it is after the nucleus that the nucleoles show 

 themselves. Most of the elements destined to a prompt destruc- 

 tion, such as the hsematia, the leucocytes, have no nucleus. If 



Fia. 42. 

 Ovum of dog. The embryon in form of a shoe sole is in rudimentary shape ; a, dorsal fissm-e ; 



h, dorsal plates ; c, clear (or bright) area ; d, opaque germiuative area ; e, membrane of 



the germinatiye vesicle. 

 The small superior figure is of natural size. The inferior figure is magnified. 



the primitive hsematia, those which show themselves first of all 

 in the embryon, are furnished with a nucleus, it is probably' 

 because they have those functions of which, at a later period, 

 they are deprived. In effect, in the embryon, at the moment of 

 the genesis of the first hsematia, there are not yet any of those 

 sanguineous glands called closed glands, in which new hsematia 

 are subsequently to be fabricated. Also the first hsematia are 

 completer elements, endowed with an intenser vitality, and 



' Ch. Kobin, Des EUmenta Anatomiques, pp. 47 — 54. 



