248 



ANIMAL BIOLOGY 



IL 



W 



m. 



AaBbCcDd 



aBCd. 



AbcD 



^1 



yn 



Fig. 167. — Diagram of the chromosome cycle of an animal. Somatic (dip- 

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

 sperm) = ABCD; maternal (from egg) = a b c d. 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 homol- 

 ogous 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. See: Fig. 189. (After Wilson.) 



