EMBRYOLOGY OF COCCIDS 115 



studied by Lecaillon ('98), no differentiation has been observed 

 among the cleavage cells until their migration to the periphery, 

 when they become the blastoderm cells. Although the point at 

 which the first cleavage cell reaches the periphery differs with the 

 species, yet in all species which have been investigated so far a 

 complete layer of blastodermic cells forms around the periphery 

 of the egg. Those cleavage cells that eventually come to lie 

 around the posterior pole of the egg where the polar granules are 

 located, become larger and clearer than their neighboring cells. 

 They then become disconnected. 



Wheeler ('89) was not so fortunate as to observe such an early 

 segregation of the primitive germ glands in Leptinotarsa (Dory- 

 phora) decemlineata. According to him, these cells arise as two 

 elongated thickenings of splanchnic mesoderm. Saling's obser- 

 vation ('07) on the development of genital cells in Tenebrio 

 molitor is in accord with those of Heider and Wheeler in Hydro- 

 philus and Leptinotarsa. The germ sells of Hydrophilus piceus 

 also arise at a much later period of development. According to 

 Heider ('89), the germ glands of this beetle are derived from the 

 inner wall of the primitive abdominal segments on either side of 

 the body. At first they are indistinguishable from the neighbor- 

 ing mesodermal cells from which they originate, but soon they 

 grow in size, and their nuclei become clearer. In the following 

 Orthoptera the germ cells have been observed to develop in 

 such a manner and at about the same period of embryonic 

 development as in Hydrophilus: 



Oecanthus niveus, Ayers ('83). 



Blatta germanica, Heymons ('90-'91); Wheeler ('99). 



Periplaneta orientalis, Nusbaum ('66). 



Xiphidium, Wheeler ('93). 



The sex cells of the Hymenoptera have been derived from 

 mesoderm cells in much the same fashion as in the Orthoptera. 

 This statement agrees with the researches of Grassi ('84) for 

 Apis, of Carriere and Burger ('97) for the mason-bee (Chalico- 

 doma), of Petrunkewitsh and of Nelson ('15) for the female 

 honey-bee. 



