THE LIFE CYCLE 569 



zygotes nor derived from zygotes, but primitive haploids. While it is 

 to be expected that the cases of critically proved instances of sexual 

 reproduction will increase in both number and systematic range with 

 further investigation, even this will be far from establishing the uni- 

 versality of sexual differentiation among the Protista. Haploid (odd) 

 numbers of chromosomes in primitive species will still require a solu- 

 tion. There are three chromosomes in Trichomonas buccalis (Hinshaw, 

 1926) and five in lodamoeha butschlii, according to unpublished obser- 

 vations made in my laboratory by Dr. Dora P. Henry. 



In the absence of sex and sexual reproduction among primitive Proto- 

 zoa, this first type of a merely asexual life cycle is the only one feasible. 

 It is, however, incorporated into the second type of cycle, in which it 

 alternates in varying irregularity with sexual reproduction and may even 

 exhibit several forms with structurally different functional individuals 

 within the same cycle, as in Vlasmodimn. 



Asexual Reproduction in Alternating Binary 

 AND Multiple Fission (Type I) 



An example of the first type of the protozoan life cycle among the 

 Mastigophora is seen in Trichomonas augusta, in which asexual repro- 

 duction by binary fission prevails, but is interrupted at unknown inter- 

 vals by the formation of an eight or sixteen-celled somatella with a 

 common cytoplasm, each cell of which has its own neuromotor ap- 

 paratus. Within this plasmodium paired schizonts, temporarily joined 

 to each other by the paradesmose, ceaselessly tug at this tether until 

 they are disunited except by the common cytoplasm. Serial plasmotomy 

 releases each schizont, to start again the cycle with binary fission. There 

 is in this type of cycle no clue to sexual reproduction. 



Another example from the Rhizopoda is found in Councilmania la- 

 fieuri, usually called Endamoeba coli, in which there is an alternation 

 between a unicellular free motile phase and a multicellular encysted one. 

 During the motile phase binary fission prevails, and reversion to the 

 unicellular condition follows each mitosis. This is interrupted from time 

 to time by the encysted phase, in which, following reduction in volume, 

 the body rounds out and secretes about itself an impervious membrane 

 or cyst wall of elastin, with a differentiated exit pore closed by a plug. 

 Encystment follows feeding and the accumulation of food reserves, which 



