274 BIOLOGY OF THE PROTOZOA 



porary or permanent fusion. It would be mere presumption to 

 claim that our present state of knowledge permits an explanation 

 of these phenomena, but there is an abundance of data from which 

 working hypotheses may be deduced. 



Gametic Differences.— In Metazoa differences in gametes are 

 reduced to practically those between egg and spermatozoon. In 

 Protozoa there is no common type of difference but all gradations 

 may be found here, from apparently similar individuals to differ- 

 entiated eggs and spermatozoa. This has led to attempts to classify 

 gametes for purposes of description, into those which are similar 

 (isogametes) and those which are dissimilar (anisogametes). Similar 

 gametes, however, may be minute derivatives of adult individuals 

 — microgametes— or they may be adult individuals which cannot 

 be distinguished from ordinary asexual, vegetative individuals. 

 The latter type is represented by the vast majority of Infusoria, 

 and, as Minchin maintained, there is very little justification for 

 calling them gametes at all; yet they come together for purposes of 

 fertilization and to this extent at least resemble gametes. In the 

 majority of Protozoa fertilization involves the permanent fusion of 

 cell bodies as well as of cell nuclei and the term copulation is applied 

 to such cases. In the Infusoria fertilization involves the permanent 

 fusion of nuclei only, while the cell bodies, with few exceptions, are 

 incompletely fused and this is only temporary (Fig. 139). To this 

 phenomenon the term conjugation is given. A conjugating ciliate, 

 however, is physiologically different from a vegetative individual 

 and may be distinguished by the general designation gamont. 

 These considerations lead to the following classification: 



(a) Conjugation.— Temporary cell fusion of gamonts; permanent 

 nuclear fusion. 



(b) Copulation.— Permanent fusion of cell bodies and cell nuclei 

 of gametes. 



(a) Similar macrogametes or gam- 



A. Isogametes onts (hologametes) . 



(b) Similar microgametes. 



Gametes { J (a) Dissimilar microgametes. 



i (b) Macrogametes and microgam- 



B. Anisogametes -j e t e s. 



(c) Egg and spermatozoa (oogam- 



ogamy). 



(a) Hologametes and Conjugates.— The nearest approach to 

 conjugation of the ciliates is to be found in the fertilization phe- 

 nomena (pseudo-conjugation) of the Sporozoa, particularly in the 

 Gregarinida. Here, two gamonts (gametocytes) come together but 

 do not fuse; after the formation of a common gametocyst each cell 



