INFLUENCE OF SYNGAMY 133 



of chromosomes in the nuclei of gametes, or in the two nuclei into which 

 the zygote nucleus divides. In this reduction division the individuals 

 of each pair of homologous chromosomes are separated, one of each pair 

 going to each daughter nucleus. If, for instance, there are four pairs of 

 homologous chromosomes grouped as ka, B6, Cc, Dr/, at reduction division 

 one of each pair passes to a daughter nucleus, so that the daughter nucleus 

 may receive chromosomes in many possible combinations — -ABCD, ABCc?, 

 ABcrf, khcd, abed, A6CD, etc. In all, there may be sixteen different 

 combinations. When syngamy occurs, any one of these groups in one 

 gamete will unite with any one in the other gamete, so that the zygote 

 nucleus containing eight chromosomes will have a still larger number of 

 possible combinations, the actual number being eighty-one. 



It has been abundantly demonstrated in the higher animals and plants 

 that the hereditary characters are intimately bound up with the various 

 chromosomes occurring in the nuclei of the gametes, so that it is clear 

 that union of gametes with four chromosomes will give rise to eighty-one 

 different combinations of hereditary characters. In ordinary division 

 without conjugation all the chromosomes split longitudinally, and half of 

 each chromosome passes to each daughter nucleus, so that the hereditary 

 characters are more equally distributed to the daughter nuclei. On this 

 account, Jennings (1920) sees that the progeny resulting from conjugation 

 show a greater diversity of hereditary combinations than do the progeny 

 arising from multiplication by fission. From the point of view of survival 

 of the race, the diverse individuals resulting from conjugation will be 

 more likely to provide at least some forms which will tolerate any new 

 condition of the environment than are the more uniform individuals 

 which result from continued asexual reproduction alone. The group of 

 organisms which result from conjugation will be at a distinct advantage 

 when compared with others when changes in environment take place. 



LIFE-HISTORY OF PROTOZOA. 



The life-history of a Protozoon is one of continued growth and repro- 

 duction, which may or may not be interrupted at intervals by a process 

 of syngamy. When syngamy occurs, two ordinary individuals which do 

 not appear to differ from those which have been dividing may copulate, 

 as in Copromonas, or conjugate, as in Paramecium, after which reproduction 

 is resumed. On the other hand, it may happen that certain young 

 individuals which arise in the usual manner, and which do not appear to 

 differ from others which are destined to develop into forms like the parent, 

 become transformed into individuals of a special type. They are known 

 as gametocytes, which, when fully grown, produce a number of gametes. 



