164 



PRINCIPLES OF ANIMAL BIOLOGY 



conjugation itself is completed. Fusion of the micronuclei, however, 

 initiates a series of changes covering a long period. These processes in 

 one of the exconjugants are essentially as follows. The fusion micro- 

 nucleus divides three times (Fig. 132/-M), resulting in the formation 

 of eight micronuclei. Of these, four enlarge and become macronuclei, 

 while the other four remain micronuclei. The exconjugant then divides 

 twice (N-P), each new individual receiving one micronucleus and one 

 macronucleus. After a growth period each cell divides by fission (page 

 1G9) in the ordinary manner and at intervals of 16 to 24 hours thereafter 

 for a considerable period, when again conjugation usually occurs. The 

 part of this process which corresponds to fertilization is the exchange of 

 micronuclei and the formation of a new nucleus from the two parts. 



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Fig. 133. — Conjugating strains of paramecia: at left, single strain, no conjugation; 

 middle, two strains mixed ; right, clumps sorted out, mostly into pairs. {From Wichterman 

 in Turtox News.) 



The repeated divisions of the cells following conjugation arc to be likened 

 to segmentation of the fertilized ovum of the metazoa. 



A most interesting fact is that there are diffei'ent strains of Para- 

 mecium, so organized physiologically that members of the same strain 

 will not conjugate with one another, but all of them will conjugate with 

 those of certain other strains. When members of two strains which 

 will conjugate are mixed, they first form large clumps (Fig. 133). These 

 aggregations slowly disintegrate and after a few hours are sorted out, 

 mostly into pairs. Some biologists have been tempted to regard this 

 distinction between .strains as sex, despite the difficulty of deciding 

 which of two conjugating strains is female, which male. Since each 

 member of a pair receives a micronucleus from the other, they would 

 seem rather to be hermaphrodites (see page 166). 



Parthenogenesis. — In an earlier paragraph it was said that sexual 

 reproduction usually involves two parents and the fusion of two germ 



