some animals {e.g. Crayfish, p. 368) the sperms are non- 

 motile. 



The peculiar variety of mitosis described above, by 

 which the number of chromosomes in the sperm-mother- 

 cells is reduced by one half, is known as a reducing 

 division. 



As already stated, the ova arise from primitive sex-cells, 

 precisely resembling those which give rise to sperms. These 

 divide and give rise to the egg-mother-cells in which, as in 

 the sperm-mother-cells, the number of chromosomes is even- 

 tually doubled. The egg-mother-cells do not immediately 

 undergo division, but remain passive and increase in size 

 by the absorption of nutriment from surrounding parts : in 

 this way each egg-mother-cell becomes an ovum. Sometimes 

 this nutriment is simply taken in by diffusion or osmosis, 

 in other cases the growing ovum actually ingests neighbour- 

 ing cells after the manner of an Amoeba. I'hus in the 

 developing egg the processes of constructive are vastly m 

 excess of those of destructive metabolism. 



A\'e have seen (p. 249) that the products of destructive 

 metabolism may take the form either of waste products 

 which are got rid of, or of plastic products which are stored 

 up as an integral part of the organism. In the developing 

 egg, in addition to increase in the bulk of the protoplasm 

 itself, a formation of plastic products usually goes on to an 

 immense extent. In plants the stored-up materials may 

 take the form of starch, of oil, or of proteid substance : in 

 animals it consists, as mentioned above, of rounded or angular 

 grains of proteid material, known as yolk-granules. These 

 being deposited, like plums in a pudding, in the proto- 

 plasm, have the effect of rendering the fully-formed egg 

 opaque, so that its structure can often be made out only 

 in sections. 



