SEX IN PROTOZOA 205 



diice a resistant oocyst membrane as does that of most Coccidia. By 

 numerous divisions many nuclei are formed from the zygote nucleus, 

 the number varying with the size of tlic zygote. The cytoplasm then 

 divides into as many sporoblasts as there are nuclei (4). Sporocysts 

 next form about the sporoblasts which produce spores. Within the 

 spore cytoplasm the single nucleus divides, then only one of the 

 daughter nuclei divides, just as occurs in the oocyst of Chagasella 

 (21). Therefore, only three sporozoites are formed in each spore (5). 

 Since the spores are not confined by an oocyst membrane, they are 

 free to separate from each other. One would suppose that a single 

 spore would be able to continue the life cycle. When taken into a 

 crab, the schizogonic part of the cycle develops. When a cuttlefish 

 eats a crab, the merozoites from the crab continue the cycle in the 

 cuttlefish by developing into gamonts and gametes, which unite to 

 produce zygotes again (12 to 18). By analogy to Chagasella (19 to 

 26), we might suppose that sex determination takes place in the spore 

 in this case, rather than at the first postzygotic nuclear division. Pos- 

 sibly two of the three sporozoites in a spore are male and the other 

 female. If so, any spore could continue the life cycle in a new host. 



If we could assume that any spore with two or more sporozoites 

 contains both male and female elements, then most of the spores of 

 gregarines and Coccidia would contain both sexes. However, in the 

 life cycle of Neviatopsis ostrearum (Prytherch, 1940), each spore 

 contains only one sporozoite (Fig. S, 17). To produce an infection 

 with new gamonts, such a sporozoite would have to be able to give 

 rise to both male and female elements; otherwise male spores and 

 female spores would both have to enter a new host in order to con- 

 tinue the life cycle. Perhaps the fact that sporozoites enter a new host 

 oyster in clusters (Fig. S, 11) would help to insure that both sexes 

 would be present in an infected oyster. We are thus faced with the 

 interesting problem of the mechanism of sex determination in this 

 group of parasites. 



In typical Haemosporidia, as a rule, no sex difi^erences are visible 

 until the gametocyte (gamont) stage is reached. The gametocytes, 

 however, are distinguishable as to sex, and the gametes are strikingly 

 different, as in the parasite of malarial fever for example. Presumably 

 a mosquito must take up both kinds of gametocytes in order for fer- 

 tilization to take place in its digestive tract. After sporozoites are 

 formed and migrate to the salivary gland of the mosquito, they are 



