606 FERTILIZATION 



Fertilization in the Cnidosporidia is typically autogamous, and will 

 be dealt with under the subject of autogamy. Little is known of the 

 fertilization phenomena in the Acnidosporidia, but Crawley (1916) de- 

 scribes gamete formation in Sarcocystis murh, similar in general to 

 oogamy in Eiweria except that the microgametocyte gives rise to the 

 microgametes by a peculiar kind of nuclear fragmentation. 



Autogamy 



Autogamy, or self-fertilization, is accomplished in several ways in the 

 Protozoa, but the result in all cases is the fusion of two gamete nuclei, 

 both of which have been derived from the same parent cell. In some 

 cases the two pronuclei have been separated by cytoplasmic divisions 

 into separate cells which later fuse. In other cases the two pronuclei re- 

 main in the undivided cytoplasm and fuse after casting out part of the 

 chromatin, with or without visible meiotic reduction. 



Whatever benefit there may be to the individual or the race in ex- 

 ogamy, or cross-fertilization, in the way of renewing the vigor of the 

 protoplasm and in propagating the race, this benefit is also a property 

 of autogamy. There is no apparent reason why autogamy in these re- 

 spects should not be as efficacious as exogamy. In two respects, however, 

 the processes differ greatly. In the uniparental inheritance of autogamous 

 individuals, meiosis will shuffle and sort out whatever genes are pres- 

 ent; and if the allelomorphs are different, the resulting gametes and 

 offspring will vary in their characteristics. However, the tendency in this 

 kind of inbreeding would be overwhelmingly toward the production of 

 homozygous races. Therefore the pronuclei and the resulting offspring 

 would be less variable than in races in which exogamy brings together 

 two sets of genes from two different parents, in the production of a 

 heterozygous individual (see Jennings, 1920). From the genetic and 

 the evolutionary standpoint, it is evident that exogamy would tend to 

 produce a more heterozygous race; and, if natural selection is the criti- 

 cal factor in evolving organisms by the selection of favorable variants, 

 then the advantage of exogamy over autogamy is apparent. 



Another difference is the advantage the autogamous species has over 

 the exogamous species in accomplishing fertilization. It is obvious that 

 in exogamous species either the sexual organism or the gamete must 

 seek out and find a mate before syngamy may occur; and if the organ- 



