GERM CELLS AND THEIR FORMATION 113 



ever (Fig. 32). In some of the bony fishes germinal cells are 

 recognizable in the fifth cell generation, i.e.. in the thirty-two 

 cell stage. And in many other forms, including some of the 

 Mammalia, the germinal cells can be distinguished from the 

 somatic cells very early, even in the blastula (Fig. 59). This 

 may indicate that, although not visibly distinct, the germ 

 tissue is, after all, in reality distinct from the somatic, in most 

 if not in all forms. It would seem more consistent with the 

 present conception of development, however, to say that this 

 distinction exists only potentially and comes about as a real 

 differentiation in the developing organism, for however early 

 this differentiation may occur, a stage is always found where 

 germinal and somatic substances are contained undifferentiated 

 within a single cell and are then indistinguishable. 



The visible distinction between the gonads of different sexes 

 may occur very early. In some forms this distinction between 

 sexes can be made out in the fertilized ovum. And in many 

 forms the two kinds of gonads can be distinguished soon after 

 they are first marked out, though there is reason even here for 

 supposing that the distinction is really, though not visibly, 

 present in the fertilized egg. 



The processes involved in the later differentiation or histo- 

 genesis of the eggs and spermatozoa are collectively termed 

 oogenesis and spermatogenesis respectively. They are con- 

 veniently divided for description into three periods or phases. 

 These are (1) the period of cell multiplication, during which the 

 simple epithelial cells, or primordial germ cells, divide more or 

 less continuously, increasing the bulk of the gonad; (2) the 

 period of growth, when cell division is less rapid or altogether 

 inhibited, and the cells enlarge rapidly, the egg-forming cells 

 much more considerably than those forming the spermatozoa; 

 (3) the period of maturation, when the germ cell nuclei undergo 

 profound modifications during their last two divisions as germ 

 cells. Sometimes the terms oogenesis and spermatogenesis are 

 used to indicate only the events of this third period, which are 

 of such importance that we shall make them the subject of the 



