GENERAL ZOOLOGY 



maintained for long periods (Calkins, P. caudalum), it was observed that conju- 

 gation occurred at intervals of some 200 generations. In others (Woodruff, 

 P. aurelia), it was found that conjugation did not occur, even in many 

 thousands of generations. The details of conjugation in P. caudatiim, after 

 the two individuals have fused together in the region of the buccal grooves, 

 are shown in Figure 8.23, which should be carefully studied. The process 

 differs remarkably from the syngamy, or permanent fusion of gametes, which 

 occurs in many other protozoans as well as in higher animals. Syngamy 

 involves the permanent and complete union of two cell bodies and their 

 nuclei; conjugation consists of a temporary union and exchange of nuclear 

 material, and it is known to occur only in the Ciliophora. However, the net 

 outcome is the same in syngamy and in conjugation. The result of 

 syngamy is a single cell with nuclear contributions from two cells. In conju- 

 gation, two cells unite temporarily, and after they separate, each "exconju- 

 gant" has a nucleus of double origin. Conjugation thus appears actually to 

 be more efficient than syngamic fertilization, because the result of conjugation 

 is two cells, each with a new combination of chromosomes. In fertilization 

 the outcome is a single cell, the zygote. 



In endomixis the macronucleus and a considerable portion of the micro- 

 nuclear material disappear. From a single persistent micronucleus, new 

 macro- and micronuclei are formed. Thus endomixis involves a nuclear 

 reorganization comparable with that occurring during conjugation, but endo- 

 mixis takes place within a single individual. Endomixis and binary fission 

 are commonly regarded as asexual methods of reproduction; conjugation is 

 considered a sexual process, although the two members of a conjugating pair 

 are not obviously differentiated as male and female cells. 



In 1937, within the species Paramecium aurelia, a number of "mating types" 

 were discovered which are related to the occurrence of conjugation. In each 

 of several morphological varieties of this species two mating types were 

 recognized; individuals conjugate only with members of the opposite mating 

 type within their own variety. Similar phenomena have since been described 

 for other species of Paramecium, and for other ciliates as well. The exist- 

 ence of mating types may be taken as evidence of physiological diflFerentiation 

 between individuals, possibly related to the kind of differentiation which has 

 led to the development of sexual dimorphism in other animals. The proto- 

 zoan mating types, however, are not opposite sexes in the usual sense, as 

 indicated by the fact that the transfer of nuclear material between the two 

 individuals is mutual. 



The full significance of conjugation in ciliates is still unsettled. It is clear 

 that the resulting exconjugants are individuals with new chromosomal com- 

 plements and new characteristics, as are the zygotes which result from fertili- 

 zation in metazoans. The question that has led to endless investigation and 

 discussion for years is whether conjugation has an important physiological 

 effect upon what may be termed cell vitality, as measured by the rate of cell 

 division; upon the longevity of the race; and upon normal cell activities. 



256 



