,CHAP. in.] OF GKOWTH IN THE ANIMAL KINGDOM. 



287 



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

 cular fibre become a nervous fibre, &C. 1 



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

 the anatomical element. It is the nucleus which habitually 

 appeal's the first ; it is after the nucleus that the nucleoles show 

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

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



FIG. 42. 

 Ovum of dog. The embryon in form of a shoe sole is in rudimentary shape : a, dorsal fissure ; 



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



the germinative 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 

 1 Ch. Kobin, DCS EUmcnts Anatomiqucs, pp. 47 54. 



