88 SYNGAMY IN PROTOZOA 



two gametes are formed from a single individual in the secondary cyst. 

 In such an example there is an extreme instance of inbreeding. More 

 recently Belaf (19216, 1923) has described in detail a similar process of 

 pedogamy for another Heliozoon, Actinophrys sol (Fig. 50). A single 

 uninucleate individual encysts and divides into two daughter forms, which 

 become gametes. The nuclei undergo two divisions, one of which is 

 a reducing division in that the number of chromosomes is halved. One 

 of the products of each nuclear division degenerates. The two gametes 

 within the cyst then unite. The development is comparable with that 

 which occurs in the secondary cysts of Actinosphcerium. In the case of 

 Actinophrys sol, Schaudinn (1896a) stated that two individuals entered 

 the cyst, but doubt was thrown upon this by Distaso (1908) and Prowazek 

 (19136), who stated that the two gametes were derived from one individual. 

 Belaf has finally confirmed the statements of the latter observers. He 

 has also noted that occasionally two individuals encyst together, and that 

 each divides to form two gametes, so that four gametes occur within the 

 cyst. After the maturation divisions have taken place, the gametes 

 unite in such a way that those formed from one individual unite with 

 those from the other. In some cases, of the two gametes formed from one 

 individual, one is motile and the other not, so that a distinction between 

 male and female gametes can be drawn (Fig 50, lo). 



PARTHENOGENESIS. — Amongst higher animals it sometimes happens 

 that the ovum, which usually develops only after fertilization, does so 

 without this having taken place. It is evident from what has already been 

 explained that in such a case the nucleus will only possess half the number 

 of chromosomes that it would have had if fertilization had occurred. It 

 has been found that during the parthenogenetic development of the 

 ovum the nucleus behaves in a variety of ways, by which the double 

 number of chromosomes is regained. Another feature of parthenogenesis 

 is that, though the ovum which develops without fertilization may give 

 rise to the same type of individual as it does when fertilized, this is not 

 necessarily the case. Thus, the ova of the honey-bee if fertilized develop 

 into females, if unfertilized into males. Amongst the Protozoa, several 

 observers have attempted to establish the occurrence of parthenogenesis. 

 The most notable instance is that described by Schaudinn (1902tt) for 

 the malarial parasite, Plasmodium vivax of man. This observer supposed 

 that the female macrogamete, which usually develops only after fertiliza- 



8. Two stages in second maturation division of the gamete nuclei. 



9. Completion of second maturation division and formation of second i eduction body: the two 



reduction bodies are still present in later gamete. 



10. Mature gametes, showing sexual dimorphism: the male gamete has pseudopodia. 



11. Union of two gametes and commencing fusion of their nuclei. 



12. Zj'gotc within its cyst. 



