84 Reproduction and Life-Cycles 



some extent from that reported by Moewus. Strains of C. reinhardi, C. 

 minutissima, and C. intermedia, for example, become motile and develop 

 sexual activity in darkness as well as in light (214, 215). However, light 

 seems to be required for clumping and pairing of C. moewusi (161). 



Prior to the work of Moewus, Schreiber (213) had described -|- and — 

 strains in Goniurn and Pandorina, mixtures of different clones producing 

 zygotes in some combinations but not in others. Tests with lines started 

 from division-products of zygotes indicated that differentiation occurred 

 in the first or second postzygotic fission. 



Aside from such biochemical differentiation of similar gametes, the 

 development of minor structural differences apparently preceded the 

 evolution of marked gametic dimorphism. Among the gregarines, for 

 example, primitive anisogamy may involve differences in size of the nuclei, 

 differences in shape, and slight differences in size of the gametes. This 

 trend culminated in the development of small microgametes, resembling 

 spermatozoa in their low cytoplasmic content, and relatively large macro- 

 gametes containing appreciable amounts of stored food. Such extreme 

 differentiation is characteristic of certain Sporozoa (Coccidia, Haemo- 

 sporidia) and Volvox (Fig. 2. 18). 



Pedogamy 



In this process, attributed to Actinophrys and Actinosphaerium, a 

 single organism encysts and then divides into two or more "gametocytes." 

 After meiosis occurs, the resulting gametes undergo syngamy. In repeat- 

 ing earlier observations on Actinophrys sol, Belaf (18) described a reduc- 

 tional division in each gametocyte, followed by degeneration of one of 

 the two haploid nuclei. Fusion of the uninucleate gametes was then fol- 

 lowed by encystment of the zygote. The occurrence of syngamy in Helio- 

 zoida seems unquestionable but the validity of "pedogamy" may be less 

 certain. It has been suggested that, as in certain Foraminiferida (185), 

 two associated "gametocytes" secrete a common cyst membrane. However, 

 such an interpretation is not supported by Belaf's data. 



Autogamy 



The older literature (109) contains numerous descriptions of au- 

 togamy. In a typical account, the nucleus of an encysted amoeba divides 

 and each daughter nucleus undergoes meiosis. The haploid nuclei then 

 fuse in pairs. Or, fusion may be preceded by degeneration of all except 

 two haploid nuclei, so that only one synkaryon is produced. Believing 

 that such cases are open to more plausible explanations, protozoologists 

 generally had considered autogamy a highly dubious process. 



The cjuestion was reopened by Diller's (70) report of autogamy in 

 Paramecium aurelia. Autogamy, followed by meiosis of the synkaryon, 

 was reported shortly afterward in Phacus pyrnm (157), although this 



