236 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



it arises from the lateral edges of the germ band as two flaps which spread 

 medially toward each other and ultimately fuse into a single membrane. 

 It is said to be of ectodermic origin. This membrane at first serves as a 

 gliding surface beneath which the splanchnic mesoderm progresses 

 medially. At first in close contact with each other, later the membrane 

 and the mesoderm separate near the lateral edges of the germ band, thus 

 forming the first pair of lateral blood sinuses. During blastokinesis the 

 portion of the provisional dorsal closure lying between the blind ends 

 of the stomodaeum and proctodaeum becomes free at the edges and grows 

 around the yolk, forming a temporary mid-gut covering. The anterior 

 and posterior ends distad of the stomodaeum and proctodaeum remain 

 unchanged until some time after blastokinesis, when they probably 



n A A (TX 



proct 



Fig. 158. — Locusta. Successive steps in blastokinesis. Sagittal sections, {am) 

 Amnion, {ch) Chorion, {dc) Provisional dorsal closure, {do) Secondary dorsal organ. 

 {gut) Mid-gut. {proct) Proctodaeum. {ser) Serosa, {stom) Stomodaeum. 



degenerate, being replaced by the definitive epidermis. Middorsally the 

 provisional dorsal membrane is fused with the amnion which now forms 

 the second provisional dorsal closure of the embryo. The splanchnic 

 mesoderm now grows dorsad and separates the first dorsal closure from 

 the amnion. The former provisional dorsal closure then degenerates, 

 leaving the inner layer of the splanchnic mesoderm as a temporary mid- 

 gut cover until the definitive mid-gut epithelium is formed. 



Shortly before the beginning of blastokinesis the amnion and serosa 

 become secondarily attached to each other at the cephalic end of the 

 embryo and then rupture there (Figs. 157, 158). The fate of the 

 embryonic envelopes resembles that already described for the dragonfly. 



Locusta migratoria shows a marked blastokinesis, the embryo turning 

 along the posterior pole of the egg through an angle of 180 deg. (Fig. 158). 



