96 THE SFBTSTG GRAIN-APHIS OR "gBEEN BUG.*' 
Ail these changes can be readily observed with a hand lens by 
holding the egg up to the light. At low temperatures (below 40° F.) 
these changes take place slowly. 10 or more days being required 
for the egg to turn black, if the temperature is near the freezing 
point. The black coloration is apparently due to a pigment in the 
shell: the green color, to the developing embryo. 
At deposition the egg is coated with a viscous substance which 
hardens in a few days, fixing the egg firmly to the object upon which 
it rests. 
T-iere are but two membranous coverings to the ripe egg, the 
chorion, or shell covering, and the vitelline membrane. 
The chorion is a rather tough, leathery, homogenous membrane 
which under a hand lens appears smooth and shining. With a com- 
pound microscope very faint lines or cracks can be someiimes ob- 
served on the surface, although usually the surface appears perfectly 
smooth, with no markings whatever. 
The vitelline membrane is structureless, colorless, and trans- 
parent. Under the vitelline membrane is the peripheral layer of 
protoplasm. This layer is very thin and very finely reticular. It is 
continuous over the surface of the egg, the cleavage cells lodging in 
it to form the blastoderm. 
Internally the egg consists chiefly of a compact mass of yolk 
granules, supported within the meshes of almost clear protoplasm. 
The yolk granules are structureless and subspherical in shape and 
vary greatly in size, ranging from 0.0027 mm. to 0.013 mm. in 
diameter. 
At the posterior pole of the egg is a large, dense, almost spherical, 
granular mass. These granules are 0.0019 mm. in diameter, are 
almost uniform in size, and the central area apparently takes the stain 
slightly as though it were a chromatirdike substance. As previously 
stated, we have termed this mass the ovarian yolk. It is evidently 
not homologous to the secondary yolk of the parthenogenetic em- 
bryos. The ovarian yolk is formed approximately at the same time 
as the formation of the main yolk mass of the egg, while in the case 
of the parthenogenetic forms of aphidids the secondary yolk enters 
the egg as the blastoderm is forming. It appears also, from our 
material, that this oviiriaii yolk is not exactly homologous to the 
"pole disk" described and observed by Hegner (1908) 3 as we have not- 
been able to observe that it affects the nuclei in any way. nor have 
we found any cells which we think correspond to his '"pole cells.'' 
The function of this granular mass seems to be the nourishment of 
the developing ovaries, and we have therefore called it ovarian yolk. 
It is not entirely used up in the early stages of embryonic growth, 
and remains in close proximity to the developing ovaries throughout 
the later stages. 
