THE EGG. 47 



Eggs arc usually small in proportion to the size of the 

 parent; but in many minute forms (i.e. Pulex, Pediculus, etc.) 

 they are proportionately much larger. In shape eggs are 

 either spherical or oblong. In some there are radiating append- 

 ages at one end, as in those of Nepa and Ranatra ; or they are 

 provided with a single stalk, as in Chrysopa^ Cynips, and 

 Ophion. 



The eggs of most Hymenoptera, Diptera, and many Coleop- 

 tera arc usually cylindrical ; those of Lcpidoptera are more 

 generally spherical. The eggs of the Mosquito are laid in a 

 boat-shaped mass, which floats on the surface of quiet pools, 

 while those of the Chrysopa, or Lace-winged Fly (Fig. 55), are 

 supported on long pedicels. 

 They are almost invariably 

 laid near or upon objects des- 

 tined to be the food of the Fig. 55. 

 future larva. Thus the Copris, or "Tumble-bug," places its 

 egg in a ball of dung which it rolls away to a secure place; 

 the Flesh-fly oviposits on meat ; and all vegetable-feeders lay 

 their eggs on the food-plant where the larva, upon its exit 

 from the egg, shall readily find an ample supply of food. 



The posterior end of the egg is more often the fixed one, and 

 it may thus be distinguished from the anterior pole. In the 

 eggs of some Diptera and Orthoptera. the ventral side of the 

 embryo, according to Gerstaecker, corresponds to the convex 

 side of the egg, and the concave side of the latter corresponds 

 to the dorsal region of the embryo. 



The surface of the chorion, or egg-shell, which is dense and 

 brittle, is often covered by a mosaic-work of more or less regu- 

 lar facets. In many small eggs the surface is only minutely 

 granulated, or ornamented with ribs and furrows, as in those 

 of many Butterflies. 



The Micropyle. On the anterior end (though sometimes 

 at both ends) of the egg is one or more pores of exceeding 

 minuteness, through which the spermatozoa (more than one 

 of which, according to Darwin, is requisite to fertilize an 

 ovule) enter to fertilize the egg-contents. In some cases 

 these micropyles are scattered over the whole surface of the egg. 

 Fig. 56 a represents the micropyles of Nepa cinerea, consisting 



