254 SPERMATOGENESIS AND OOGENESIS 



noted that the whole cell is not used up in the formation 

 of the sperm, a part of it being cast off in the form of a 

 protoplasmic globule as the mature form is assumed. 



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

 precisely resembling those which give rise to sperms. Very 

 soon however the behaviour of the sex-cells in the ovary 

 distinguishes them from the similar cells of the spermary. 

 Instead of actively dividing they remain passive and increase, 

 often enormously, in size, by the absorption of nutriment 

 from surrounding parts. Sometimes this nutriment is simply 

 taken in by osmosis, in other cases the growing ovum actually 

 ingests neighbouring 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-framed 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) 



