EMBRYOLOGY OF COCCIDS 113 



Following the closure of the dorsal wall of the embryo along 

 the dorsal midline by the lateral growth of the ventral plate, 

 and with the further ingrowth of the vaginal invagination, the 

 germ cells are carried further cephalad and thus again come into 

 contact with the colony of the symbiotic organisms. But this 

 time they collect beneath, instead of encircling the organisms, 

 as was the case at first. Throughout the larval and adult stages 

 the germ cells and the colony of symbiotic organisms retain their 

 relative positions. The former is always found beneath or 

 ventrad of the latter as though they were under the protection 

 of the latter. 



The foregoing account of the history of the germ cells in the 

 mealy bug is essentially applicable to that of Lecaniodiaspis. 

 One of the striking differences is that in the case of Lecanio- 

 diaspis the germ cells begin to migrate at a much later period 

 than in Pseudococcus macdanieli. The number of germ cells that 

 migrate toward the symbiotic organisms in the case of Lecanio- 

 diaspis is very much smaller than in the case of Pseudococcus 

 (fig. 89). 



Inasmuch as the origin and subsequent history of the germ 

 cells in the cottony cushion scale is somewhat different from 

 what has already been given for the mealy bug and Lecaniodiaspis, 

 a short account for this species may be added here. 



In the cottony cushion scale the germ cells become noticeable 

 at the posterior pole of the egg just as in the mealy bug. They 

 are larger, clearer, and their nuclei stain much more feebly than 

 those of the other cells around them. Although there is no doubt 

 that they, like those of the mealy bug and Lecaniodiaspis, are 

 able to migrate, the movement is not well marked on account of 

 the comparatively short distance that separates the blastodermic 

 layer from which the germ cells are derived and the colony of 

 symbiotic organisms. The germ cells of the cottony cushion 

 scale come into contact with the organisms earlier than in the 

 other two species, since the migration is a short one. This fact 

 of the early association of the germ cells with the symbiotic 

 organisms, and also the posterior position of the latter, led 

 Mecznikow ('66) to a statement that the symbiotic organisms 



