EMBRYONIC ENVELOPES, DORSAL ORGANS, BLASTOKINESIS 63 



Paratenodera (Hagan, 1917). In some insects the only movement is that 

 occasioned by growth in length of the embryo which then remains from 

 beginning to end on the ventral side of the egg, the head directed toward 

 the cephalic pole. 



Some writers beheve that blastokinesis may be due to purely phys- 

 iological causes, the embrj^o having "acquired the habit of moving to a 

 different part of the egg where the yolk is as yet unpolluted by the 

 waste products as a result of metabolism. ..." TireUi (1931) and 

 others, however, have expressed the view that blastokinesis is due to 

 mechanical and spatial conditions dependent upon the manner of growth 

 of the embryo structure of the insect body and the constricted space 

 within the egg. 



In apphed entomology it has been shown (Baker, 1921) that the time 

 of revolution is extremely important in interpreting the results of experi- 

 ments in the control of apple aphids, the overwintering insect undergoing 

 blastokinesis in the spring before a marked rise in temperature, which 

 would be fatal to the insect at this time. 



Blastokinesis has been experimentally studied in the grasshopper 

 Melanoplus. Here it consists of a reversing of the longitudinal axis 

 followed by a revolution around this axis. Slifer (1932a) shows that this 

 change of position is accomplished by vigorous movements of the embryo 

 itself. These movements originate as contraction waves running along 

 the lateral borders of the dorsally incomplete abdomen and passing 

 rapidly to the head. With the closure of the dorsal wall and the forma- 

 tion of the dorsal vessel they seem to become resolved into the heartbeat, 

 as was suggested by Nelsen (1931). In overwintering eggs, diapause 

 interrupts incipient blastokinesis as well as other developmental activities, 

 these processes being resumed immediately after the end of the diapause 

 period. 



Although the embryonic membranes are usually ruptured, Slifer 

 reports one positive case in which blastokinesis was initiated and partly 

 completed without the rupture of the serosa, showing that the contraction 

 of the embryonic membranes cannot be the primary cause of revolution. 

 Hence, in the grasshopper the revolution of the embryo must be due to its 

 own movements. In sections of embryos of this age Slifer (1934) found 

 unicellular, nonstriated, spindle-shaped fibers in the position of the future 

 abdominal muscles. She suggests that these cause the movements 

 (striated muscles do not appear until nine days later). 



Concerning the necessity of revolution Slifer (1932a) reports four 

 cases in which it failed to occur and yet the embryos developed more or 

 less normally but were incapable of hatching. But TirelU (1931) reports 

 that the occasional failure of blastokinesis in the silkworm egg invariably 

 results in death. However, in the latter case blastokinesis brings the 



