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. 273, B, 

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

 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 



Fig. 291. Male sexual apparatus of a bark-beetle, deferens ;"' ff, 'seminal vesicle 



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 band. 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 

 293. section of sphinx membrane (called amnion, but not 

 y oiki g r^rou s m mem d 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. 293-298, the 



