350 



ZOOLOO 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 (rliab- 

 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. 



11 



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

 si, \ as deterens ; ho, testis ; II, seminal vesicle ; ag, 

 ductus ejaculatorius. After Graber. 



Fig. 297. t, testis; 17, vas 

 deferent ; g, seminal vesicle 

 o f Achtta 'carnpestris. 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 

 Fis.298.-iZn of s P hin X membrane (called amnion, but not 

 In u!e yoik" ^TerouTme"^ homologous with that of vertebrates) 



bi -ane; am, amnion;/*, outer, peels off from the primitive band lUSt 

 m, inner germ-layer. - 1 



as the appendages are budding out, so 

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

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

 a glove. As seen in the accompanying Figs. 298-303, the 



