214 THE PROTOZOA 



three years later in the sentence, " the exhaustion of the rejuvenating 

 power (i.e. the exhaustion of the initial potential of vitality) becomes 

 the stimulus for the formation of the sexual products." 1 



A. PHENOMENA OF CONJUGATION 



With our present knowledge it is impossible to say that conjuga- 

 tion is absent in any group of Protozoa, and until the life-cycle of 

 every genus is fully known, the conservative but logical view which 

 Biitschli expressed in regard to flagellates, is the most acceptable. He 

 says : " I, personally, am inclined to the view that the significance of 

 this process in the life of these organisms is so general and deep- 

 reaching that the failure to observe it in certain groups up to the 

 present time is no reason for considering it absent." 2 Nevertheless, 

 the observations which have been recorded show the greatest diversity 

 in the process, the variations passing from extremely simple fusion, 

 which only by stretching the meaning of the term can be called 

 sexual, to the highly differentiated male and female organisms where, 

 as in the Metazoa, fertilization is followed by cleavage. 



Stages in the development of so-called sexual reproduction may be 

 considered as follows : 



1. The permanent or temporary union of similar adult cells 

 (Isogamy) (Sarcodina, Sporozoa, Mastigophora, Ciliata). 



2. The union of individuals apparently similar in all respects save 

 size (Anisog&my) (Sarcodina, Mastigophora, Ciliata). 



3. The union of reduced individuals. Swarm-spores (Isogamy or 

 Anisogamy) (Sarcodina, Mastigophora). 



4. The union of specialized individuals male and female cells 

 (Spermatozoa and eggs) (Sporozoa, Flagellidia). 



I. The permanent or temporary union of similar adult individuals 

 (Isogamy). 



Thanks to the unbroken observations of Dallinger and Drysdale, the 

 conjugation and full life-history of some of the lowest forms of Pro- 

 tozoa (monads) have been made out. All of the forms examined 

 reproduce by simple division for a few days, and then conjugate. In 

 Cercomonas longicauda Duj. (typica Kent), one of the Monadida, 

 reproduction by ordinary fission continues for two to four days, when 

 the offspring, without losing their flagella, become amoeboid and con- 

 jugate two by two (Fig. 116). The union begins with the fusion of 

 the pseudopodial processes, and, as it progresses, the flagella are 

 withdrawn. The nuclei finally unite (D\ and the product of the 

 union, or the zygote, forms a thin-skinned globular cyst (E). After a 



1 ('79), P- 199. 2 ('83), P. 778. 



