Feb. 21, 1916 
Green Apple Aphis 
961 
over, in both of these cases, and also in examinations of many isolated 
trees, the eggs were found to be much more abundant on the southwest 
sides of the trees. 
The eggs adhere so tightly to the bark that great care is needed in 
removing them, and often this can not be done without breaking them. 
On downy twigs it is impossible to remove the eggs without also removing 
some of the hairs which adhere to them. Neither alcohol nor xylol will 
dissolve the adhesive or free these hairs from the egg. 
EMBRYOLOGY 
General embryology. —The substance of the unfertilized egg is very 
clearly divided into two areas. The first, comprising nearly all the space 
included within the vitelline membrane, is filled with the food yolk, which 
consists of homogeneous granules enmeshed in a fine network of proto¬ 
plasm. The second area, filled with smaller granules, which the writers 
are calling the “ovarian yolk,” following Webster and Phillips (17, p.95), 
is rather spherical in shape and lies at the posterior pole. Surrounding 
these two bodies is a very narrow layer of peripheral protoplasm, the 
periplasm or “Keimhaut blastem” in which the blastoderm will form 
later. The egg is included within two envelopes, the vitelline membrane 
and the chorion. 
At the time of deposition the fertilized egg appears like the sterile egg. 
In a very short time, however, the production of cleavage cells com¬ 
mences, and the formation of the blastoderm is initiated. This begins 
at the anterior pole and progresses most rapidly in that region, but in a 
short time covers the entire yolk, with the exception of the posterior end, 
where it lies in contact with the ovarian yolk. A portion of the cleavage 
cells do not migrate to the periphery, but remain in the yolk to become 
yolk cells. 
Invagination commences by a thickening of the blastoderm in its area 
of contact with the ovarian yolk, brought about by the division of the 
blastoderm cells along this area. 
At the end of about five days the germ band attains a condition in 
which it rests or hibernates till early spring. In this resting stage the 
embryo occupies a position in the center of the egg, with its cephalic 
portion directed toward the posterior pole. The posterior half of the 
abdominal region is flexed dorsad in such a manner as to include the 
ovarian yolk. Segmentation is well advanced, and the formation of the 
appendages has begun. The stomatodeum and proctodeum are present, 
while the formation of the mesenteron has begun. The genital rudiments 
are separated into two groups, although the ovarian yolk is not yet 
divided. At the posterior pole lies an organ composed of a single layer 
of cells surrounding a pear-shaped orange body without structural char¬ 
acters. This has been designated by Webster and Phillips (17, p. 98) 
as the “polar organ/' 
