xi] APHIDS 



some, and a small one containing only the two autosomes. 

 This smaller cell never divides again ; it persists for a time 

 and then degenerates. The larger secondary spermatocytes 

 proceed almost immediately to the second division, in which 

 all three chromosomes divide equationally, producing two 

 equal spermatids each with three chromosomes. The final re- 

 sult of the process, therefore, is that every spermatogonium 

 gives rise to two spermatids each with an X-chromosome, 

 and to a small degenerate cell which disappears. Since all 

 the mature eggs contain two autosomes and an X-chromo- 

 some, all the fertilised eggs have four autosomes and two 

 X-chromosomes, and thus all develop into females. 



An interesting problem arises with regard to the cause of 

 the throwing out of one X-chromosome in the polar body 

 of the male-producing eggs. In Phyllaphis the same female 

 may produce both female- and male-producing eggs, but in 

 Phylloxera the males and females are produced from differ- 

 ent parents, and in some species the two kinds of eggs are of 

 different sizes. In Phylloxera caryaecaulis investigated by 

 MORGAN the life-cycle is as follows. The "stem-mothers," 



Stem-mother Stem-mother 



I I 



parthenogenetic egg parthenogenetic egg 



winged female (female-producer) winged female (male-producer) 



large parthenogenetic egg small parthenogenetic egg 



female male 



sexual egg sperm 



stem-mother 



