234 FOUNDATIONS OF BIOLOGY 



We have now surveyed the germ cell cycle from fertil- 

 ized 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 

 which has just been considered. Obviously this fertilized egg 

 (zygote) contained chromosomes, half of which belonged to 

 the egg and therefore may be termed MATERNAL, and half of 

 which were derived from the sperm and thus are PATERNAL. 

 When the zygote divided by mitosis to form the body and germ, 

 every cell received a set of chromosomes, directly derived 

 from this original set in the zygote. It logically follows, 

 and all observations indicate, that each and every cell, both 

 of the soma and of the germinal tissue, possesses a set of chro- 

 mosomes, half of which are of maternal and half of paternal 

 origin in other words are direct lineal descendants of the 

 combined set formed at fertilization. So it happens, that 

 each body cell really has a double set (DUPLEX GROUP, DIPLOID 

 NUMBER) of homologous chromosomes and the same is 

 true of the germ cells until maturation. Then at synapsis 

 maternal and paternal chromosomes pair and, after the re- 

 duction division, the secondary spermatocytes and oocytes 

 and the gametes themselves have a single set (SIMPLEX 



GROUP, HAPLOID NUMBER). (FigS. 124A, 126.) 



Thus far we have emphasized chromatin and, in particular, 

 chromosome reduction as the main purpose of the compli- 

 cated maturation phenomena. The question now arises: la 

 this chromatin distributed so that all the gametes receive the 

 same heritage? 



All the evidence at hand indicates not only that chromo- 

 somes differ qualitatively one from another but also that 

 the various parts (CHROMOMERES) of each chromosome are 



