58 BIOLOGY 



the animal cells, they almost immediately after fertilisa- 

 tion lose their reproductive quality and again become 

 vegetative. 



Among animals and plants we have seen that the 

 reproductive cells pass through a process of preparation 

 for fertilisation in which the stages are almost identical. 

 The main feature of this process is a reduction of the 

 amount of chromosome material in the nucleus, and is 

 known as the process of chromatin-reduction or matura- 

 tion. This is brought about by a complex series of 

 changes in the chromatin, always accompanied by 

 karyokinetic division. A full account of this process 

 would be very interesting, as it lies at the root of several 

 important questions, but space forbids even a superficial 

 account. 



CHAPTER XI 



ONTOGENY 



EVERY living organism begins life as a single cell, and 

 ultimately reaches a more or less varying degree of com- 

 plexity according to the race to which it belongs. The 

 study of the stages between these two extremes is known 

 as Ontogeny; this is, the various transformations that 

 each organism must pass through before it reaches its 

 full complexity. The early stages in the development 

 of an organism up to the point at which its specific 

 characteristics begin to be well defined is known as 

 Embryology. 



If the embryo becomes self-sustaining and independent 



