634 Gro. W. TANNREUTHER, 
derm or serosa, amnion and germ band are continuous until after 
the differentiation of the organs have begun. 
The resulting completely developed embryo in both instances is 
apparently the same, in size, structure and appearance. 
The amnion in the parthenogenetic forms can be recognized 
before the invagination is complete, while in the sexual developing 
embryo the amnion can not be distinguished until after the germ 
band is completely separated from the uninvaginated part of the 
blastoderm and when the differentiation of the germ band begins. 
The vitelline membrane and chorion are present only in the sexual eggs. 
In the adult aphid, where nearly all of the embryos have reached 
their later stages of development, the reproductive organs undergo 
a remarkable change. The specialized ovarian glands of the end 
chamber have entirely disappeared and the follicular wall of the 
oviduct becomes a thin tough membrane with a few disorganized nuclei. 
This change is most marked in the sexual females, where it is 
very difficult to find the end chamber or ovaries of the reproductive 
organs after the eggs have reached their growth. The abdomen 
of the adult female becomes a hypodermal-like sac filled with eggs, 
being separated only by the thin membranous oviduct wall which 
can be recognized under high power. This deteriation of the re- 
productive organs was found in all the species studied, being especially 
well marked in the sexual generation where the yolk is completely 
formed before embryonic development begins. 
Brief summary of results. 
The somatic number of chromosomes, — six —, is a generic 
characteristic. The chromosomes vary in size, four large and two 
small. This number and size of chromosomes is constant in both 
the sexual and parthenogenetic forms. 
In the male, the six univalent ran unite end to end 
in pairs in the early prophase of the first spermatocyte division and 
form two large and one small bivalent chromosome. There is a 
short resting period between the first and second spermatocyte 
division. Each spermatid receives three chromosomes, two large 
and one small. No accessory chromosome is present. The first 
division separates bivalent and the second division divides univalent 
chromosomes. 
The six chromosomes at the beginning of the growth period in 
the sexual ova pass into the resting stage and the reduced number 
268  — 
