GERM CELLS IN THE ARTHROPOD A 113 



laid in this manner. Gravity apparently has no 

 influence upon the development, since eggs in a state 

 of nature occupy all positions with respect to 

 this factor without becoming altered in any way. 

 Only one case has come to the writer's attention 

 of an influence of gravity in insect development — 

 the eggs of the water beetle, Hydrophiliu- atterimus. 



, p r d X a 



Fig. 35. — A diagramatic drawing of Calligrapha bigsbyana clinging to 

 the under side of a willow leaf and showing the orientation of the 

 egg in the ovarian tubule and after deposition, a = anterior ; d = 

 dorsal ; p = posterior ; r — right side ; x = place where egg was 

 marked with India ink as means of orientation after removal from 

 leaf. 



according to Megusar (1906), develop abnormally if 

 the cocoon in which they are laid is inverted. 



The events that precede the establishment of the 

 primordial germ cells in chrysomelid beetles may be 

 described briefly as follows : The egg, when laid 

 (Fig. 36, A), consists of a large central mass of yolk 

 globules (y), among which are very fine strands of 

 cytoplasm ; a thin peripheral layer of cytoplasm, the 

 ''keimhautblastem" of Weismann (JxJihl), a delicate 

 vitelline membrane (v.m.), a chitinous shell, the 

 chorion, and a nucleus consisting of the egg nucleus 



