EMBRYOLOGY OF COCCIDS 89 



division of the blastoderm cells as well as by a further migration 

 of cleavage cells from within. 



The point at which cleavage nuclei, or cells as they are often 

 called, reach the surface of the egg varies in different groups of 

 insects. In Muscidae Graber (79) found the first arrival of 

 cells at the posterior end of the egg, while in Pieris Bobretzky 

 ('78) observed the appearance of the first blastoderm at the 

 anterior end. "Wheeler ('93) described the first blastoderm cells 

 on the ventral side, while Heider ('88) stated that the blastoderm 

 in Hydrophilus was first formed around the middle of the egg 

 as a transverse girdle, somewhat nearer the posterior pole and 

 that the development occurred last at the poles. Again, accord- 

 ing to Nelson, the cleavage cell first reaches the cortical layer 

 on the ventral side near the cephalic pole in the egg of the honey- 

 bee. In the winter egg of the aphid, Melanoxanthus (Ptero- 

 comma salices), according to Tannreuther ('07), all of the 

 blastodermic cells spread uniformly over the entire surface 

 except at the posterior pole of the egg. Therefore, I agree with 

 Nelson ('15) that the point at which the first cleavage cells 

 reach the surface has little significance so far as the formation 

 of the blastoderm is concerned. 



The condition of the egg at the time the cleavage process has 

 ceased among the cells within the egg and the blastoderm forma- 

 tion is completed, is shown in figure 80. At the poles and sides 

 the blastoderm is similar in appearance. A short distance 

 within the blastoderm is another loose layer of cells. This is 

 irregular in shape, and the nuclei are much clearer and coarser 

 than those of the blastodermic cells. These are the so-called 

 yolk cells of Will ('84), and are no other than the cleavage cells 

 that failed to migrate to help form the blastoderm. At about 

 the time invagination occurs at the posterior end of the egg, 

 these cells move toward the periphery and become closely apposed 

 to the blastoderm cells. 



