THE LIFE CYCLE 571 



in the cyst are stored in a large central glycogen vacuole. The glycogen 

 is transformed into chromatoidal structures of unknown composition, 

 staining deeply and formed on the surface of the glycogen vacuole. 

 These progressively disappear as mitotic divisions ensue. 



Soon after encystment is completed, a series of mitotic nuclear di- 

 visions occur, resulting in two, four, eight, and sixteen-cell stages, 

 rarely thirty-two-cell, and in one observed instance approximately a 

 sixty-four-cell stage, thus running the rhythm of normal cell division 

 in a metazoan egg. Plasmotomy, however, does not attend the nuclear 

 divisions. Cyst formation, in this instance, serves the function of assimila- 

 tion and growth. Measurements of cysts in the one, two, four, eight, 

 and sixteen-cell stages show a slight progressive increase in diameter. 



Excystment occurs normally in the bowel, as shown by the occurrence 

 of cysts free from glycogen or chromatoidals, with reduced numbers of 

 nuclei from fifteen down. It can also be followed in fresh stools, as 

 the small mononucleate amoebulae escape singly out of the exit pore. 

 Excystment is a form of asexual reproduction, of budding, or progres- 

 sive multiple or serial fission. In this type of life cycle we find an alterna- 

 tion of a unicellular free phase' with reproduction by binary fission, with 

 the formation of a multicellular encysted somatella, with reproduction by 

 multiple fission and a return to the unicellular motile phase. 



Alternation of Asexual and Sexual Reproduction (Type II) 



The second major type of the protozoan life cycle is that in which 

 asexual and sexual reproduction alternate. It may or may not be accom- 

 panied by sexual dimorphism, as exhibited by differential reaction to 

 aniline stains in Nma, by structural differentiation of gametes in Eimeria, 

 of gametocytes in Plasmodium, or of conjugants in Vorticella. It seems 

 probable that sex has become a genetic characteristic of the individual 

 throughout the whole cycle, in all life cycles having sexual reproduc- 

 tion, even though structural features indicative of sexual dimorphism 

 cannot be detected. 



From the biological point of view, it is unfortunate that the life 

 cycles of parasitic Protozoa have been arranged, in illustrations, in se- 

 quences as parasitic cycles, rather than biological life cycles. They are 

 usually designated as beginning with the infection of the host, or in 

 the case of a parasite with two hosts with that of the primary host, or 



