206 SEX IN MICROORGANISMS 



ready to infect a new host. One wonders if a single sporozoite could 

 give rise to both kinds of gametocytes in a new vertebrate host. In the 

 case of Babesia (Dennis, 1932) the gametocytes are not morphologi- 

 cally differentiated in the vertebrate host. Just where and how sex 

 determination takes place in the Haemosporidia can only be guessed 

 at, but one might suppose that the mechanism would be like that of 

 the Coccidia. 



Cnidosporidia. In the Myxosporidia the available evidence indi- 

 cates that the trophic stages are commonly diploid and that meiosis 

 takes place at some stage during sporogony. However, according to 

 Naville (1930a), meiosis takes place at a very early stage in sporogony 

 with evident differences between the sexes, whereas according to 

 Georgevitch (1935, 1936) and Noble (1944) all nuclei of a myxospo- 

 ridian are diploid except the gamete nuclei, which are formed at the j 

 last nuclear division during spore formation. Since these nuclei seem ' 

 to be exactly alike (Fig. W, 15 to 17), there is no sex difference, and 

 therefore no sex determination. Furthermore, the interpretations of 

 Noble indicate that the two gamete nuclei, as sister nuclei, unite to 

 produce a diploid zygote in a process of autogamy. Autogamy is 

 likewise indicated for the Actinomyxidia, Microsporidia, and Hap- 

 losporidia as previously indicated. Just what biological advantages 

 are derived from such a process is difficult to imagine. 



The Effect of the Host on Life Cycles 



It is interesting to speculate about what determines the shift 

 from one stage of a life cycle to the next. In gregarines, for instance, 

 what induces gametocyst formation? In Coccidia, what causes the 

 cessation of schizogony and the onset of gametogony? In the Hae- 

 mosporidia, what determines that certain merozoites will become 

 gametocytes (gamonts) instead of schizonts? Are such changes of 

 activity due to some inherent cyclic tendencies, or do they occur in 

 response to conditions or stimuli provided by the host? 



Cleveland's remarkable discovery of the inducement of sexuality 

 in the intestinal flagellates of the wood-feeding roach, Cryptocercus 

 pimctiilatus, by the molting hormone of the host naturally raises the 

 question whether parallels can be found elsewhere. Possibly similar 

 conditions have been described where different stages in the life cycle 

 are correlated with developmental stages of the host. For example, 



