98 
the entire layer is then one cell in thickness. The blastoderm, how- 
ever, does not cover the surface of the ovarian yolk. 
Not all of these cleavage cells reach the surface; many remain 
behind, increasing in number within the yolk. These latter cells are 
indistinguishable from those of the blastoderm. Figs, la and lb 
represent two of these cells magnified 845 diameters, showing them 
to be star-shaped masses of protoplasm with a large oval coarsely 
granular nucleus, more often with a large clear area of nuclear sub- 
stance around the mass of chromatin granules. 
At the posterior pole, about the ovarian yolk, the blastoderm be- 
gins to thicken and to invaginate (Stage 2, PL III, figs. 2-4). This 
is the beginning of the germ band. At this stage {Stage 2) some of 
the yolk cells apparently pass into the ovarian yolk. Tannreuther 
(1907, p. 631) states that the thickening of the blastoderm is caused 
by the rapid division of the blastoderm cells of this particular part. 
We find, in addition, that some of tlie cells from the interior of the 
egg migrate to the posterior pole to assist in this process. Each of 
the cells of this thickened area is very elongate, and, from a surface 
view, now has a somewhat polygonal shape, with a large coarsely 
granular nucleus. The growth of the cells of the germ band carries 
the ovarian yolk toward the center of the egg (see PI. Ill, fig. 4)- 
The part of the blastoderm that invaginates first becomes the posterior 
part of the embryo, and that part that invaginates last becomes the 
anterior portion. 
In Stage 3 (PL IV, fig. 1) the germ band is ready to free itself from 
the blastoderm. The former is now cone-shaped, the base being 
closed by the ovarian yolk. 
When the germ band releases itself from the blastoderm, it leaves 
behind what we have termed the "polar organ:" A cluster of cells 
embedded within a mass of protoplasm. These cells soon group 
themselves into a more or less spherical mass, with a less dense 
vacuolar area at the center (see PL IV, fig. 4). In later stages this 
central area appears denser and structureless, as though filled with 
a fluid, and is of a yellow color, not taking the stain, and opening 
directly upon the surface of the egg. For these reasons we suggest 
that it may be an organ of excretion. When development ceases in 
the fall, this body is still present. 
What was formerly the blastoderm now becomes the serosa. The 
cells are much more widely spaced now and this wall is much thinner, 
except at the anterior pole, where the cells are apparently crowded 
more closely than before. Some of these cells often show large 
vacuoles on the side toward the yolk. 
At Stage 4 (PL IV, fig. 2) the germ band is completely submerged 
in the yolk, has assumed a tubular shape, and is near the center of 
the egg. The walls are of uniform thickness and composed of a com- 
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