SEXUAL AND ASEXUAL REPRODUCTION 241 



It is probable that in most other unicellular organisms a 

 similar cell union occurs under some conditions. As already 

 described, it occurs in the malarial organism in the cycle that 

 takes place in the mosquito; Fig. 25 j. The cell union that 

 takes place is a true sex union, since there is a clear dis- 

 tinction of male and female cells. While such a union of cells 

 has by no means been found in ail unicellular organisms, it 

 has been found in many, and we know that it is quite widely 

 distributed. The studies of recent years particularly have 

 shown one large group of unicellular organisms called the 

 Sporozoa, which live as parasites on various hosts, and show 

 a cell union resembling that of malaria. Another example of 

 this will be given here in illustration of the phenomenon of cell 

 union among single-celled animals. 



Monocystis. In the earthworm may be found living a 

 unicellular parasite known as Monocystis. This animal (see 

 Fig. 112 A) is a single elongated t cell possessing a nucleus, but 

 with no other visible organs. The animal has no locomotor 

 organs, although it does have a slight power of motion. Its 

 method of reproduction involves a cycle, in which a cell union 

 alternates with a formation of spores without cell union, but 

 in a complicated manner. When ready to multiply, two indi- 

 viduals fuse with each other and become surrounded by a 

 covering or cyst; Fig. B. Inside of this cyst both of the indi- 

 viduals divide. First the nucleus divides into many parts 

 (see Fig. C), and later the rest of the protoplasm divides and 

 collects around the pieces of the divided nuclei, thus making 

 many small cells. Now the new cells from one of these indi- 

 viduals unite in pairs with the cells from the other. This step 

 occurs within the cyst, but is shown separated from it in Figure 

 Dj a, by and c, and it constitutes the cell union proper. When 

 the cells fuse together their nuclei unite, forming a single nu- 

 cleus, c, called the fusion nucleus, which divides into eight 

 parts, at e, after which the whole cell divides into eight elon- 

 gated cells (see /) known as sporozoites. Meantime a hard 



