INTRODUCTION 



9 



beyond an asexual multiplication has led to a widespread belief 

 that simple division is the only means of reproduction. How- 

 ever, in the Protozoa which have been thoroughly investigated, 

 it has been found that an asexual cycle (schizogony) is frequently 

 followed by a sexual cycle (sporogony). Presence of a compli- 

 cated life cycle, involving an alternation of generations, has 

 thus been established for some single-celled creatures (see Fig. 

 29). 



Specialization of Gametes. — In its most characteristic form, 

 sexual reproduction involves the complete fusion of two special- 

 ized gametes to form a zygote (Fig. 6). In some instances, 



Fig. 6. — Diagram of fertilization and cleavage. It is assumed that maturation 

 of the egg has been completed before the entrance of the spermatozoon. {After 

 Sharpc). 



however, the fusing cells are termed isogametes because they 

 seem to be alike in form and function. Though isogametes have 

 considerable significance in hypothetical discussions of the origin 

 and differentiation of sex cells, they are of relatively infrequent 

 occurrence among animals. Even in some instances in which 

 fusing gametes are indistinguishable in size the two react differ- 

 ently to cytological stains and thus give evidence of a probable 

 differentiation even though morphological differentiation is 

 lacking. 



The gametes of most animals show two distinct lines of differ- 

 entiation. The enlargement of the cell through accumulation of 

 yolk or deutoplasm is characteristic of macrogametes or egg 

 cells. In some instances, the ovum does not contain all of the 

 stored food material but is accompanied by special storage cells 

 as yolk cells or follicle cells. Special protective envelopes or 



