350 



ZOOLO&T. 



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

 them with a tough coat. 



The external parts consist of the ovipositor (Fig. 278, B, 

 and Fig. 281), which is formed of two pairs of spines {rhab- 

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

 guide (Figs. 278 and 281, eg), a triangular flap guarding the 

 under side of the opening of the oviduct. 



Fig. 296.— Male sexual apparatus of a bark-beetle, 

 si, vas deferens ; Ju), testis ; U, seminal Yesicle ; ag, 

 ductus ejaculatorius. — After Graber. 



Fig. 397.— <, testis: v, vas 

 deferens ; g, seminal vesicle 

 of Acheta campestris.— Alter 

 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 hand. 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 

 membrane (called amnion, but not 

 homologous with that of vertebrates) 

 peels off from the primitive band just 

 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. 298-303, the 



Ficr. 298.— Section of Sphinx 

 embryo, the germ immersed 

 in the yolk ; 5, serous mem- 

 brane; am, amnion; h, outer, 

 ?«., inner germ-layer. 



