568 THE LIFE CYCLE 



budding type breaks up this body into functional individuals of the 

 ancestral type. Although nuclear division is essential in the accomplish- 

 ment of this cycle, it does not initiate it. This is shown strikingly in the 

 Polymastigophora, in which the entire neuromotor complex of centro- 

 some, blepharoplast, flagella, undulating membrane, and axostyle of 

 the individual is duplicated by new growth, accompanied by extensive 

 dedifferentiation of the parental equipment before the nuclear phe- 

 nomena of mitosis ensue. Asexual reproduction is thus profoundly an 

 organismal phenomenon involving a rejuvenation of the organelle sys- 

 tems of the body of the individual. 



This type of life cycle seemingly exists without any evidence of sex, 

 sexual reproduction, or sexual dimorphism. Efforts to establish sexuality 

 on the basis of the relative size of supposedly male and female individuals 

 and upon interpretations of behavior are will-o'-the-wisps of wishful 

 thinking. The only basis is gametogenesis, verified by reduction of the 

 diploid to the haploid number of chromosomes, and fertilization, with 

 the resulting return to the diploid state. 



The juxtaposition or even fusion of motile individuals among flagel- 

 lates and rhizopods may occur when adverse conditions or internal states 

 induce an adhesive periphery; sometimes cannibalistic feeding of rhi- 

 zopods resembles fusion; and changes in position from divergence to 

 lateral contact in sister schizonts among Mastigophora resemble conjuga- 

 tion, all of which evidence is never to be accepted as sexual behavior 

 unless confirmed by critical cytological evidence. 



The not uncommon opinion that sex is an inherent characteristic of 

 organisms and that sexual reproduction is to be expected in all animals 

 and plants and even in bacteria, is as yet without convincing cytological 

 evidence among the more primitive forms. It has, however, been clearly 

 demonstrated in the Sporozoa, Ciliata, Foraminifera, and the Volvocidae, 

 all representatives of the more highly evolved Protozoa. The present 

 evidence, negative though it be, lends support to the view that sex 

 was evolved in the Protozoa, perhaps independently in the different 

 classes. It may well be that its origin rests ultimately on differential 

 metabolism within the species, leading in time to more favorable condi- 

 tions for permanent fusion of gametes, though this alone makes no 

 provision for gametogenesis. The fact that some flagellates and rhizo- 

 pods have an odd number of chromosomes suggests that they are not 



