THE CONTINUITY OF LIFE 249 



Throughout the animal kingdom, wherever sexual reproduction 

 occurs, phenomena which can be interpreted as nuclear reduction 

 have been observed in the formation of gametes. In some of the 

 Protozoa this seems to be merely an extrusion of a certain amount 

 of chromatin, but since whenever chromosomes can be observed 

 and counted the process has been found to follow in principle 

 essentially the same lines described above, we have every reason 

 to believe that it is never a haphazard mass reduction, and that 

 the ripe gametes emerge with a definite chromatin heritage, rela- 

 tively simple as this may be in the lowest forms. 



5. The Chromosome Cycle 



We have now surveyed the germ cell cycle from the fertilized 

 egg through the germ plasm in the adult to the gametes again, 

 but before proceeding to consider the details of the fusion of egg 

 and sperm — the fertilization process — it may clarify matters 

 to glance back to the chromosome condition in the fertilized egg 

 at the beginning of the cycle that has just been considered. 



Obviously this fertilized egg (zygote) contained two groups of 

 chromosomes, one of which belonged to the egg and therefore 

 may be termed maternal, and one which was derived from the 

 sperm and thus is paternal. When the zygote divided by mitosis 

 to form the body and germ, every cell received two groups of 

 chromosomes directly derived from these two original groups 

 in the zygote. It logically follows, and all observations indicate, 

 that each and every cell, both of the body and of the germinal 

 tissue, possesses two groups of chromosomes, one of maternal and 

 one of paternal origin — in other words, direct lineal descendants 

 of the combined set formed at fertilization. 



So it happens that each body cell really has a double set (diploid 

 number) — two complete sets — of chromosomes, and the same 

 is true of the germ cells until maturation. Then at synapsis corre- 

 sponding (homologous) maternal and paternal chromosomes pair 

 and, after the maturation divisions the gametes have a single set 

 (haploid number). (Fig. 167.) 



Thus far we have emphasized chromosome reduction as the 

 main result of the complicated maturation phenomena. The 

 question now arises: Is this chromatin distributed so that all the 

 gametes receive the same heritage? 



As already stated, the evidence indicates not only that chromo- 



