330 



ZOOLOGY. 



out as the eggs pass out of the oviduct, thus surrounding 

 them with a tough coat. 



The external parts consist of the ovipositor (Fig. 373, B, 

 and Fig. 276), which is formed of two pairs of spines (rliab- 

 dites) adapted for boring into the earth ; and of the egg- 

 guide (Figs. 273 and 276, eg), a triangular flap guarding the 

 under side of the opening of the oviduct. 



.-lo 



I'ig. 291. — Male Pexual apparatus of a bark-beetle. 

 8l^ vas deterene ; Jw, resile ; bl, seminal vesicle ; ag^ 

 ductus ejaculatorius. — After Graber. 



Fig, 292.— <, testis; «, vas 

 deferens ; g. seminal vesicle 

 of Acheta campestris. — After 

 Gegenbaur. 



There is a remarkable uniformity in the mode of develop- 

 ment of the winged insects. In general, after fertilization 

 of the egg, a few cells appear at one end of the egg ; these 

 multiply, forming a single layer around the egg, this layer 

 constituting the blastoderm. This layer thickens on one 

 side of the egg, forming a whitish patch called the primitive 

 streak or land. The blastoderm molts, 

 sloughing off an outer layer of cells, 

 a new layer forming beneath ; the skin 

 thus thrown off is called the serous 

 membrane ; the second germ-layer 

 (ectoderm) then arises, and a second 

 Fig. 293.-Section of Sphinx membrane (called amnion, but not 

 inti.ryo'iilirB'irmlm'' homologous with that of Vertebrates) 



brane;am., amnion; A, outer, peels off frOm the primitive band lust 

 »i, inner germ-layer. ^ r ^ 



as the appendages are budding out, so 

 that the body and appendages of the embryo insect are en- 

 cased in the amnion as the hand and fingers are encased by 

 a glove. As seen in the accompanying Figs. 293-298, the 



