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 246, eg), a triangular flap guarding the 
under side of the opening of the oviduct. 
Fig. 291.—Male sexual apparatus of a bark-beetle, ae Pe wade 
ai, vas deterens ; ho, testis ; 02, seminal vesicle; ag, of Acheta campestris.—After 
ductus ejaculatorius.—After Graber. 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 band. The blastoderm molts, 
sloughing of an outer layer of cells, 
a new layer forming beneath ; the skin 
thus thrown off is called the serous 
membrane; the second germ-layér 
(ectoderm) then arises, and a second 
_Eig 208._Section of Sphing ae (called amnion, but not 
tmiicevlkye setout men. Homologous with that of vertebrates) 
ee ee 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 
