184 Ltfe and Death, Heredity and Evolution 



mother, forming a free cell or gamete. When two of these 

 gametes meet, they unite; thus the original number of 

 chromosomes is restored. 



Only a few of the Protozoa have been examined with suf- 

 ficient thoroughness to reveal this process of reduction and 

 recombination in the chromosomes, and in some the chromo- 

 somes are so numerous, minute and crowded that just what 

 occurs cannot be directly determined. But the cases al- 

 ready worked out, scattered as they are through the dif- 

 ferent classes of the group, show that the process is one 

 of general occurrence, here as in the higher organisms ; they 

 make it possible to recognize the occurrence of reduction 

 even when the chromosomes cannot be counted. Just be- 

 fore mating, the nuclei, both in the Protozoa and in higher 

 organisms, go through the two successive divisions (in the 

 infusoria, owing to special conditions, three), known as the 

 maturation divisions. It is in one of these as a rule that 

 the reduction in number occurs, through the distribution of 

 half the chromosomes to one nucleus, half to the other. In 

 most cases these two (or three) divisions are distinguishable 

 from all others by marked peculiarities connected with the 

 reducing process ; and these make it possible to recognize 

 the reducing divisions even when the number of chromosomes 

 cannot be counted. Whenever two (or three) peculiar divi- 

 sions occur in rapid succession just before the nuclei are 

 ready to mate, we may be practically certain that in these 

 the reduction in the number of chromosomes has occurred. 

 Such divisions we saw in the cases of autogamy (page 134 

 and Figure 41 ); in our examination of the preparations 

 for mating in the infusoria (Figures 46 to 50) ; they occur 

 indeed almost universally in preparation for mating. All 

 such divisions indicate a process of reduction in number 



