98 PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



may be formed within the cyst, in tilhna, only four, and these are all 

 alike, and, except for the smaller size, similar to the parent organism. 

 In many cases only two daughter individuals are formed within the 

 cyst, a fact showing that it is not a long step from the process of 

 simple division to that of such so-called spore formation, and tillina 

 and colpidium are examples illustrating the transition from the one 

 mode of reproduction into the other. Tillina rarely varies from the 

 formation of four spores, and then only to revert to the apparently 

 ancestral mode of simple division. Colpidium, on the other hand, has 

 progressed farther toward obligatory spore formation, and not infre- 

 quently forms eight spores within the temporary cyst. Other forms of 

 ciliate infusoria form a varying number of spores; in some, as in 

 Holophrya irmltifilius, a great number of swarm spores are developed 

 in the cyst, each similar to the parent. It is a question whether such 

 reproductive elements are entitled to the name spore, for they are not 

 formed by the simultaneous fragmentation of the mother organism, 

 but by repeated division, the cleavages following one another in rapid 

 succession; in some cases, indeed, as in tillina, the divisions follow so 

 closely upon one another that the two planes of division are sometimes 

 seen at the same time, and this activity is followed by a period of rest 

 lasting for from twelve to twenty-four hours or longer, according to 

 the vitality of the individual. If this is not simultaneous, it is very 

 close to it, and the process in these ciliates must be due to the same, or at 

 least to similar, physiological causes that bring about spore formation 

 in other cases. 



Spore formation, apart from the spores that are formed in prepa- 

 ration for fertilization, is uncommon among the protozoa and is found 

 chiefly in the one group — sporozoa — which gets its name from this 

 method of reproduction. In many of the flagellates, however, it 

 seems to be a method of reproduction which follows conjugation. 

 Thus, in Tetramihis rosti'atus and Cercomonas longicamla a cyst is 

 formed immediately after conjugation of two similar cells, and within 

 the cyst the protoplasm fragments into hundreds of minute flagellated 

 organisms. In these cases the ordinary method of reproduction is by 

 cell division, the spore formation appearing to be a special method that 

 follows upon fertilization (Fig. 67, p. 155). 



It is in the group of the sporozoa that we find the highest develop- 

 ment of the spore-forming power, and here it has been found necessary 

 to distinguish between the spores that are formed sexually, i. e., after 

 fertilization, and those that are formed asexually, for they differ both 

 in structure and in function. The spores that are formed after fer- 

 tilization are protected by firm and resisting coverings, and are able 

 to live outside of the body of the animal in which they are definitive 

 parasites; the other type of spores, formed asexually, have no such 

 coverings and cannot live apart from the host. With these various 



