THE LIFE CYCLE 567 



death. Asexual reproduction may be interjected into the midst of any 

 one of these processes, resulting in from one to many repeated genera- 

 tions of functional individuals. Not infrequently these individuals are 

 heterogonous, with marked differences in structure from the parent, as 

 for example in the larval stages of the Trematoda. 



This alternation of sexually and asexually produced generations is 

 widely distributed in the living world, ranging from some of the lower 

 algae to the Quints. The ease with which regeneration occurs after 

 mutilation and with which experimental asexual multiplication of func- 

 tional individuals may be imposed upon the genetic individual is indica- 

 tive of the fundamental organic basis of asexual reproduction, perhaps 

 as a corollary of the still more fundamental capacity of growth on the 

 part of the organism. 



The Protozoa, from the evolutionary point of view, are of excep- 

 tional interest among the phyla, since it is among these primitive organ- 

 isms that most of the basic biological properties, structures, and functions 

 of the organism have had their evolution. Within these microcosms all 

 of the basic functions of living must be performed. As one surveys 

 their diversities and complexities of pattern, one is impressed with the 

 evidence that among these minute organisms, adapted to so many eco- 

 logical niches and exhibiting so many types of behavior, a vast deal 

 of evolutionary experimentation has been enacted. It is among the 

 Protozoa and Protophyta that the following have been evolved: nuclear 

 structure, sex, sexual dimorphism, sexual reproduction, mitosis, chromo- 

 somes, gametogenesis, histogenesis, multicellularity, sex and somatic 

 cells, asexual reproduction by the various methods of binary and multiple 

 fission, budding and sporulation, and the beginnings of the organization 

 of organ systems. Varying combinations and sequences of these evo- 

 lutionary accomplishments are exhibited in the diverse patterns of life 

 cycles to be detected among the Protozoa. Cycles of comparable type, 

 in some instances apparently independently of one another, have emerged 

 to a varying degree in the different classes and orders of Protozoa. 



These cycles fall into two major groups. The first is the simpler and 

 the more primitive. It consists merely of recurrent rhythms of homo- 

 geneous asexual reproduction, in which mitosis produces a multicellular 

 (= multinuclear) body of from two to many cells, forming a Plas- 

 modium, coenobium, sporocyst, or cyst. Fission of binary, multiple, or 



