THE LIFE CYCLE 581 



necessary third mitosis of the pronuclei in the conjugants. The evidence 

 for chromosome reduction in the conjugants (Calkins and Cull, 1907) 

 is inconclusive, because of the small size and the large number of the 

 chromosomes in the three divisions prior to the formation of the zygote. 

 A cytological examination of chromosome number during conjugation, in 

 some ciliate with a small number of large chromosomes, may throw 

 light on this problem. Both pre and post-zygotic alternatives should be 

 explored. 



The asexual cycle proper of Paramecium is one of oft-repeated simple 

 binary fission, prior to which the two-celled somatella may grow, and 

 by nuclear division become a four-celled one for a brief period. This 

 cycle is one of indefinite duration. 



This survey and interpretation of life cycles among the Protozoa ex- 

 hibit the basic similarity of this fundamental characteristic of organ- 

 isms among them to those emergent among the Metazoa. Peculiar to 

 the Protozoa is the absence of sexual reproduction in the life cycles of 

 the more primitive forms, among which it appears that they live a 

 haploid life and that sexual reproduction has not as yet been evolved. 

 Rare cases among Metazoa and the Metaphyta of the seeming absence 

 of sexual reproduction are obviously secondary phenomena, but this 

 interpretation is less defensible for the primitive Protozoa. 



In the higher Protozoa, as in the Metazoa, the life cycle includes 

 maturation, fertilization, cleavage to a multicellular stage, histogenesis 

 of organelles, asexual reproduction with resulting functional individuals 

 of differing structure in the difi'erent asexual phases, sexual dimorphism, 

 adolescence, gametogenesis, senescence, and death. 



The emphasis so generally placed upon the unicellular phase of the 

 Protozoa, as against all Metazoa, tends to obscure their basic similarity 

 in life cycle to that of the Metazoa, and thus to minimize the biological 

 significances of the varied evolutionary accomplishments which have oc- 

 curred in this primitive phylum. Similarities in biological phenomena 

 are the bases on which an integrated concept of the evolution of life 

 can be erected. 



Literature Cited 



Biitschli, Otto. 1876. Studien iiber die ersten Entwicklungsvorgiinge der 

 Eizelle, die Zelltheilung und die Conjugation. Abhl. senckenb. naturf. 

 Ges., 10: 213-452, 15 pis. 



