302 ANIMAL BIOLOGY 



The egg and sperm each carry a definite number of chromosomes 

 and consequently after fertilization the zygote contains a double 

 set. For each chromosome contributed by the sperm there is a 

 corresponding, or homologous, chromosome contributed by the 

 egg. In other words, there are two chromosomes of each kind 

 which may be considered as pairs. When division of the zygote 

 takes place each chromosome splits into two chromosomes, so that 

 each daughter cell receives a daughter chromosome derived from 

 each of the original ones. Since all the cells of the organism are 

 lineal descendants by similar mitotic cell divisions, all of its cells 

 contain the double set of chromosomes — half paternal and half 

 maternal; and since the primordial germ cells have a similar origin, 

 they also have a double set of chromosomes. But during the mat- 

 uration process synapsis occurs : that is, homologous chromosomes 

 of paternal and maternal origin unite in pairs — the process of 

 fertilization which gave rise to the individual being consummated 

 in the ripening of its own germ cells. But this union is only tem- 

 porary; during the maturation process the maternal and paternal 

 chromosomes of each synaptic pair are separated and one of each 

 (though very rarely all of the same maternal or paternal set) passes 

 to the daughter cells - - segregation occurs. Thus each mature 

 germ cell, or gamete, contains one member of every chromosome 

 pair and the number of chromosomes is reduced one-half. (Figs. 

 167, 189.) 



It has been assumed that the genes for alternative characters 

 segregate in the formation of the gametes of hybrids so that a single 

 gamete bears one and not both genes of a pair of allelomorphs. 

 That is the genes, which come together in the zygote that forms 

 the hybrid, separate again in the formation of its own gametes. 

 This is just what cytological studies show. Chromosome behavior 

 exactly parallels the typical behavior of the ' Mendelian ' gene, 

 because after synapsis, during spermatogenesis and oogenesis, 

 each chromosome of paternal origin separates from the correspond- 

 ing chromosome of maternal origin. Moreover, since the genes 

 similarly situated on homologous maternal and paternal chromo- 

 somes are homologous genes, or allelomorphs, it follows that 

 homologous genes are segregated in separate gametes during mat- 

 uration — the two members of a pair of allelomorphs pass to 

 different gametes. This is the basis of the so-called purity of the 

 gametes. 



