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TEXT-BOOK OF ENTOMOLOGY 



regarded as the gastrula-invagination of insects. The lower (inner) 

 layer thus arising afterwards spreads out under the entire primi- 

 tive band (Fig. 509, B and C), the edges of which become bordered 

 by the growing amnion-fold. (Korschelt and Heider.) 



In certain forms the primitive band arises from several separate rudiments 

 which afterwards unite. Thus in Musca and Hydrophilus the anterior and pos- 

 terior ends develop first, and in Hydrophilus the procephalic lobes originate 

 independently of the rest of the band. In the Aphides, also, according to Will, 

 these lobes arise independently, afterwards uniting with the primitive band. 



D 



FIG. 512. Diagrams illustrating- the movements and envelopes of the embryo of Xiphidium : D, 

 the sta<re of the shortened embryo on the the dorsal yolk, ff, embryo returning to the ventral sur- 

 face. F, embryo nearly ready to hatch ; ch, chorion ; b. /<, blastoderm membrane ; *>, serosa ; inrl 1 , 

 outer indiisium : iin/~, inner indusium ; ind* + um, inner indusium and amnion fused; am, 

 aiiinion ; im/ 1 c. cuticle of the inner indusium ; inrf* ft, granular secretion of the inner indusium ; 

 inn. H, amniotic secretion ; r, yolk ; cl, columella; yb, primitive band. 



Division of the embryo or primitive band into body-segments. Mean- 

 while the primitive band grows at the expense of the yolk, spread- 

 ing out more and more over its surface, until in certain cases 

 (Coleoptera, Diptera, Siphonaptera, and Trichoptera) it lies like a 

 broad ribbon over the yolk, so that the two ends nearly meet on the 

 dorsal side. By this time it becomes divided by transversely im- 

 pressed lines into segments, which correspond to those of the larva 

 and adult. The first of these segments is divided into two broad 



