EXPLANATION OF PLATE XIX. 



Diagram of chromosome-cycle of Phyllaphis co-iueni. 



At the top, chromosome group of stem-mother with four autosomes and two 

 A'-chromosomes. From this three lines are represented as diverging, lead- 

 ing on the left to a female which will produce a sexual egg, in the middle 

 to a female which will reproduce parthenogenetically, and on the right 

 to a male. 



2nd and -^rd lines, metaphase groups and diagrammatic anaphases of three 

 eggs, of which the left and middle will produce females, the right a male. 

 In the female-producing eggs the Jf-chromosomes divide equationally ; in 

 the male-producing they pair and separate, so that the male has only one 

 (4th line). 



$th line, on the left, metaphase group of first polar division of the sexual egg, 

 all chromosomes paired. Below this, anaphase of first, and telophase of 

 second polar division, leaving three chromosomes in the egg. 



5/// line, on the right, metaphase group of first spermatocyte division, with 

 paired autosomes and single A'-chromosome. Below this, diagrams of the 

 first (unequal) and second (equal) spermatocyte divisions, leading to 

 spermatozoa with three chromosomes and a small degenerate cell with 

 two. Union of egg and spermatozoon restores the number six in the 

 stem-mother. 



