98 THE SPEIXG GBAIX-APHIS OS " GBEEN BUG. 



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the entire layer is then one cell in thickness. The blastoderm, how- 

 ever, does not cover the surface of the ovarian yolk. 



Xot 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, PI. Ill, 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 the 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 (PI. IV, fig. 1) the germ band is read}' 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 

 t'lernselves 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 waU 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 (PI- 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- 



