STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 



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derm is finally produced. This stage of the embryo is known as the 

 blastula. 



^-Ast. 



Pig. 34. — F'ormation of the blastoderm and the germ band. Fk., Cleavage nuclei; 

 Bl., blastoderm; Kst., germ band. {After Escherich.) , 



The blastoderm thickens by cell division in one region forming 

 the primitive streak or germ band which presents in surface view an oval 

 or elongated area along the ventral face of the egg. Soon a groove 



Fig. 35. — Diagrammatic sagittal sections to illustrate invagination of germ band 

 in Calopleryx. a., Anterior pole; ac , amnion cavity; am., amnion; b., blastoderm; 

 d., dorsal; g., germ band; h., head end of germ band; p., posterior pole; s., serosa; v., 

 ventral; y., yolk. {Folsom after Brandt.) 



appears in the centre of the germ band due to invagination of the 

 blastoderm. The lips of the groove close over the invaginated portion, 

 producing an outer \a.yer, the ectoderm or edoblast and the.inner\a,yer, 



