ORIGIN OF THE INDIVIDUAL 



235 



JIL 



AaBbCcDd 



aBCd. 



AbcD 



FIG. 126. Diagram of the chromosome cycle of an animal. Somatic (diploid) 

 chromosome number assumed to be eight. Paternal chromosomes (from sperm) = 

 A B C D; maternal (from egg) = a b c d. I, union of nuclei of gametes, each with a sim- 

 plex group (haploid number) of chromosomes, in the zygote at fertilization to form a 

 duplex group (diploid number) of chromosomes. II, III, IV, somatic divisions or divi- 

 sions of germ cells before maturation (duplex groups of chromosomes). V, synapsis, 

 involving pairing of homologous paternal and maternal chromosomes to give the 

 haploid number of paired chromosomes. VI, reduction division separation of pairs 

 into single chromosomes again. VII, two gametes, with simplex groups (haploid 

 number) of chromosomes; there are 14 more possible combinations of the chromosomes, 

 or types of gametes, which are not shown. (After Wilson, slightly modified.) 



