256 SPERMATOGENESIS AND OOGENESIS LESS. 



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 

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

 division but remain passive and increase, often enormously, 

 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 osmosis, 

 in other cases the growing ovum actually ingests neigh- 

 bouring cells after the manner of an Amoeba. Thus in the 

 developing egg the processes of constructive are vastly 

 in excess of those of destructive metabolism. 



We saw in the second lesson (p. 33) that the products of 

 destructive metabolism might 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, as in Nitella (p. 214), 

 of oil, or of proteid substance : in animals it consists of 

 rounded or angular grains of proteid material, known as 

 yolk-granules. These being deposited, like plums in a 

 pudding, in the protoplasm, have the effect of rendering the 

 fully-formed egg opaque, so that its structure can often be 

 made out only in sections. When the quantity of yolk is 

 very great the ovum may attain a comparatively enormous 

 size, as for instance in birds, in which, as already mentioned 

 (p. 69), the " yolk " is simply an immense egg-cell. 



When fully formed, the typical animal ovum (Fig. 61) 



