214 REPRODUCTION 



tation of cell cycles in a series of unicellular and multicellular 

 organisms (Fig. 110). Type 1 of this tabulation is the cycle of 

 a simple protozoan in which there is no process of conjugation. 

 The cycle in this instance consists merely of asexual reproduction 

 by cell division. Changes in the rate of division may occur, and 

 there may be encystment; but these may be omitted for the 

 purposes of the present discussion. There is nothing to com- 

 pare with the cycle in a many-celled organism (Type 6), except the 

 cells and their division. 



In Type 2 is shown a cycle that occurs in a few species of proto- 

 zoa, in which there is a periodic reorganization of the nucleus, 

 either by autogamy or by endomixis. Autogamy may be com- 

 pared with the nuclear reorganization involving nuclear fusion 

 that occurs during conjugation but not with the part of conju- 

 gation that involves a union of two cells, since it occurs within 

 the limits of a single cell. Endomixis is a nuclear reorganization 

 within a single cell but without nuclear fusion. The life cycle 

 of such a protozan consists of a series of cell divisions with 

 occasional nuclear reorganization. 



Type 3, in which conjugation occurs, is the most representative 

 protozoan cycle. The species reproduces asexually by cell division, 

 and at times sexually by conjugating cells, the gametes {cf. p. 174), 

 which unite in syngamy to form a single cfll, the zygote} The 

 reproductive cycle in such a protozoan thus consists of the asexual 

 reproduction of new individual cells by division, and occasionally 

 sexual reproduction of new cells by syngamic union or conjugation. 

 The gametes are isogamous, and hence there are no structural differ- 

 ences between them that indicate sexual differentiation, although 

 it is conceivable that functional differences may exist. 



There are, however, many species among the protozoa, as might 

 be shown by a Type 3a, in which the gametes are anisogamous and, 

 therefore, called " male " and " female " cells because they 

 resemble the spermatozoa and ova of metazoa. Hence, the 

 essential feature of sex, which is the production of male and 

 female gametes, is recognizable in the protozoa. The life cycle of 

 such species differs from that of Type 3 only in the production of 

 anisogametes. 



* It will be recalled that conjugation by temporary union and mutual 

 exchange of nuclear material, which occurs in ciliates like Paramoecium, is an 

 exceptional form of syngamy even among protozoa {cf. p. 187). 



