HERITAGE OF THE INDIVIDUAL 289 



ABCD 



abed 



JIL 



AaBbCcDd 

 V 



aBCd 



AbcD 



VI 



FIG. 146. Diagram of the chromosome cycle of an animal. Somatic (diploid) 

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

 ABCD; maternal (from egg) = abcd. I, union of nuclei of gametes, each with a simplex 

 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 divisions of 

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

 pairing of homologous paternal and maternal chromosomes to give the haploid num- 

 ber of paired chromosomes. VI, reduction division separation of pairs into single 

 chromosomes again. VII, two gametes, with simplex groups (haploid number) of chro- 

 mosomes; there are 14 more possible combinations of the chromosomes, or types of 

 gametes, which are not shown. See Fig. 147. (After Wilson, slightly modified.) 



