114 THE SMALLEST LIVING THINGS 



When Does a Young Protozoan Become an Adult? 



Conjugation in any series is not possible until there has been 

 approximately two months of continued metabolic activity. This 

 indicates that protoplasmic differentiation must have progressed 

 to a stage corresponding to what we call sexual maturity in 

 metazoa, when fusion of germ cells becomes possible. In Uro- 

 leptus mobilis there is no structural or visible indication of this 

 differentiation, but in Foraminifera, in Radiolaria, in nearly all 

 Sporozoa, and in many plant flagellates, a similar stage of sexual 

 maturity in the life cycle is indicated by definite division phe- 

 nomena which differ from the customary method of division 

 and differ also from the meiotic phenomena but which occur only 

 at this period. In Foraminifera, Radiolaria, and Gregarines, 

 minute cells termed gametes, are formed from the nuclei and 

 portions of the cytoplasm of the parent organism (Fig. 44, page 

 77). Such gametes fuse two by two, and the zygote thus formed 

 gives rise to one or more vegetative individuals with optimum 

 vitality. There is considerable evidence to indicate that nuclear 

 fusion in ciliates is a reminiscence of such an ancestral gamete 

 brood. 



Differentiation into Male and Female 



In ciliates, with the exception of the Vorticellidae, there is 

 nothing in the structure of the conjugants which enables us to 

 speak of one as male and of the other as female. The same is 

 true in all protozoa where isogametes (similar gametes) are 

 formed. Yet the fact that two apparently identical cells ap- 

 proach and fuse may be interpreted as an indication that some 

 difference, which may be called sexual, exists between them. In 

 other cases, particularly in the Volvox group among plant flagel- 

 lates and in Coccidia among the Sporozoa, protoplasmic differen- 

 tiation resulting from continued metabolism follows two definite 

 paths, one leading to large forms with sluggish, or no movement, 

 and abundant reserves of nutriment; the other leading to a 

 gametocyte in which food reserves are slight and which gives 

 rise to a multitude of minute, active microgametes. Thus in 

 protoplasmic differentiation the larger form, or macrogamete, 

 is equivalent to an egg cell and the minute, active forms to 



