100 THE SPRING GRAIN-APHIS OR 
nitely segmented into more or less spherical masses, separated by 
thin walls, each area or mass containing a number of yolk granules 
and from one to several cells. The polar organ is now almost spher- 
ical, with a central, pear-shaped area of dense, structureless, non- 
staining matter of a yellowish color, and an anterior opening. Al- 
though this evidence is insufficient it possibly indicates that the func- 
tion of this organ is excretory. The embryo reaches this stage of 
development about the third day, under favorable conditions of 
temperature. 
In Stage 7 (PL VI, fig. 1) the embryo has changed its position so 
that from a side view it has the form of a reversed figure 6. The 
portion that in the preceding stage was folded upon itself ventrally 
has reversed its position and folded back dorsally. The ovarian 
yolk is now in the region of the first abdominal segments. It is in 
contact with the embryo, and the group of cells that separated it 
from the embryo in the preceding stage has assumed almost a spheri- 
cal form, and a more posterior position, forming the genital organs 
later on. 
The three primary regions, cephalic, thoracic, and abdominal, are 
now sharply marked. Each region is distinctly segmented. The 
cephalic region has 5 segments indicated, the thoracic 3, and the 
abdominal 9, the last abdominal being relatively quite large. There 
are now 15 conical appendages. The antenna} arise from the pos- 
terior margin of each cephalic lobe. The labrum is between and slight- 
ly anterior to the antennae. The mandibles are nearer the median 
plane than the fundaments of the maxillae and the labium. The next 
three pairs of appendages represent the first, second, and third pairs 
of legs. Plate V, figure 2, represents a surface view of stage 7, show- 
ing the embryo straightened out and the position of the appendages. 
All of these appendages are evaginations of the ectoderm, cross-sec- 
tions showing an external layer of ectoderm cells and an inner layer 
of mesoderm cells. 
The stomodseum (PI. VI, fig. 1) appears now as a simple invagina- 
tion of the ectoderm, the posterior wall of the labrum forming its 
anterior wall. The proctodeum has not yet appeared, The salivary 
glands (PL VI, fig. 1) are represented by a deep, bilobed, ectodermal 
invagination between the cephalic and thoracic regions. There is 
now a star-shaped mass of protoplasm about the nucleus of the 
ovarian yolk cells and the yolk granules are grouped around these 
cells. 
The primary yolk is grouped very much as in the preceding stage 
with the exception that the masses are smaller and do not contain as 
many nuclei. 
The polar organ is smaller than formerly, with a smaller number 
of cells. It still contains a yellowish mass and communicates with 
the outer surface of the egg. 
