580 THE LIFE CYCLE 



The life cycle of Paramecium is further complicated by the fact of 

 conjugation and mutual fertilization of the conjugants. Sexual dimor- 

 phism is not evident between the conjugants as a whole, but appears in 

 the behavior of the gamete nuclei. The migrant one is assumed to be 

 male because of its motility, and the resident one female because of the 

 lack of this quality. Dimorphism of the conjugants is structurally evi- 

 dent in Vorticella, in which the males are small and the females large. 



The biological life cycle in Favamecium starts with the zygote, formed 

 from the body of a conjugant by the fusion of the two haploid nuclei, 

 one from the immigrant male gamete and the other the resident nucleus 

 of the zgg. The cleavage nucleus thus formed utilizes the cytoplasm of 

 the tgg, as in the Metazoa, with only a small amount from the male 

 gamete. The old macronucleus in each continues (Fig. 139, I-IV) to 

 disintegrate and is soon entirely metabolized into cytoplasm as food. 

 This is the death of the soma of the conjugant, the future of which is 

 henceforth under a new genetic control. There then ensue three succes- 

 sive mitotic divisions (Fig. 139, II-VI), representing the cleavage of 

 the ^gg to an eight-celled somatella, when cleavage abruptly stops (Fig. 

 139, VI) and differentiation into four somatic and four sex cells occurs 

 by the enlargement of the nuclei of the former and an increase in their 

 chromatin. The four sex cells do not all survive. Three of them dis- 

 integrate at once, leaving a somatella of five cells, four somatic and one 

 sex cell. Then begins asexual reproduction which in two peculiar binary 

 fissions distributes the four macronuclei among the four daughter schi- 

 zonts, with an accompanying division of the sex cell or micronucleus 

 at each of the two fissions. In the diagram these two asexual fissions have 

 been included in the sexual cycle, since they are necessary to restore the 

 organism to the pattern in which regular asexual reproduction prevails. 

 They otherwise belong in the asexual period. 



The precise period in which maturation occurs in the sexual cycle is 

 perhaps undetermined. It has been generally assumed that it occurs in 

 the first two divisions of the micronucleus in the conjugant, in which 

 case its third division would be an asexual reproduction of the gamete. 

 This view does not rest upon exact chromosome count. The occurrence 

 of post-zygotic maturation in the Sporozoa suggests the possibility of its 

 occurrence in the Ciliata also. This view is further supported by the death 

 of three of the post-zygotic sex nuclei and in Paramecium by the un- 



