102 THE SPRING GRAIN-APHIS OK " GREEN BUG. 



appear to be a greater number of cells about this body at this time. 

 There is no trace of the dense yellowish center of the polar organ, 

 otherwise it resembles this body very closely. However, as we have 

 lost track of this organ in the gap between figures 1 and 2, and on 

 account of the close resemblance between it and the dorsal organ 

 of other insects, we have designated it as the latter. At a later 

 stage (PL VII, fig. 3) the dorsal organ has assumed a more nearly 

 circular shape, the mouth having almost closed, inclosing a some- 

 what pear-shaped space. At a still later stage (fig. 4) the dorsal 

 organ has released itself from the margin, migrated backward, and 

 begun to disintegrate. At length it disappears by absorption in 

 the body cavity. 



At first we were not able to note a revolution of the embryo, but 

 later studies show that such a revolution does occur between figures 

 1 and 2 of Plate VII. 



After the ninth stage the development goes on very rapidly, and 

 by the latter part of March the eggs are ready to hatch. 



During the fall of 1909 a number of eggs were collected that had 

 been deposited in October and November, and these were kept until 

 the spring of 1910 to note the time of hatching. No heavy freezes 

 occurred until the 3d of December. It was found that although 

 there was nearly a month's difference in dates of deposition there was 

 not more than four or five days' difference in the time of hatching. 

 An average of 64 per cent of the eggs hatched. We have also learned 

 that eggs will not hatch unless subjected to freezing temperatures. 



SUMMARY OF EMBRYOLOGICAL DEVELOPMENT. 



There is a large almost circular mass of ovarian yolk at the poste- 

 rior pole of the egg. 



Development begins almost immediately after oviposition, and 

 proceeds more rapidly in the region of the anterior pole until after 

 the blastoderm forms, after which growth almost ceases in this region. 



The blastoderm originates through the migration of yolk cells 

 from the interior to the surface of the egg. All of the yolk cells, 

 however, do not take part in the formation of the blastoderm, part 

 remaining behind to prepare the yolk for assimilation by the embryo. 



After the blastoderm is formed it is one cell thick and covers the 

 entire surface of the egg, with the exception of the ovarian yolk. 

 The germ band originates in the region of the ovarian yolk, where 

 it invaginates and grows downward into the egg. The germ band 

 is of the completely submerged type, the uninvaginated blastoderm 

 becoming the serosa. 



Upon leaving the surface of the egg the germ band leaves behind 

 it a group of cells embedded in a mass of protoplasm. This body 

 the junior author has termed the "polar organ." 



