OVIPOSITION AND THE STRUCTURE OF THE RIPE EGG. 2G1 



body-cavity of other Insects (Ichneumonidae) lay their eggs in the 

 body of the future host within which the embryonic and post- 

 embryonic development take place. Many Insects surround their 

 eggs with a web, others envelop them in a secretion which, in the 

 ■case of eggs laid in water, swells up as a jelly (spawn of the Culicidae 

 and Phryganeidae), but in other cases hardens into a firm capsule 

 by exposure to the air (so-called egg-case or cocoon of Periplaneta 

 and Mantis), etc. 



The eggs of Insects are usually distinguished for their large size. 

 They show great variety of form, the most prevalent being an 

 elongate oval, the long axis of the egg corresponding with the future 

 long axis of the larva. In such eggs a distinction between the 

 future dorsal and ventral surfaces is indicated by a difference in 

 curvature (Fig. 129, c? and v). 



The mature egg is enclosed within two envelopes, an inner vitelline 

 membrane (Fig. 129, dh) secreted by the egg itself, and an outer 

 chorion (ch) secreted by the epithelium of the ovarian tube. The 

 latter occasionally breaks up into two layers, the endochorion and 

 the exochorion. The vitelline membrane is usually a homogeneous 

 delicate structureless membrane, but the chorion is seldom thus con- 

 stituted. In most cases it is ornamented by the presence of a 

 network of thickened ridges and markings, which vary greatly in 

 the different genera and species. 



The chorion is pierced at one or more points (micropyles, Fig. 129, 

 m) to allow of the passage of the spermatozoa, and the modification 

 of the chorion that takes place round these micropyles often results 

 in a very complicated micropylar apparatus, round which the vitelline 

 membrane appears attached to the chorion (Fig. 129), so that both 

 membranes appear perforated at this point. 



In Insect eggs there is always a distinction between an anterior 

 and a posterior pole. The anterior pole is that which, in the body 

 of the mother, lay directed towards the head, and thus corresponded 

 to the upper end of the ovarian tube. In later stages of embryonic 

 development, the head end of the embryo always lies at this pole, 

 while the posterior end of the embryo is directed towards the posterior 

 pole of the egg. The micropylar apparatus usually lies at the anterior 

 pole of the egg. 



A cap of glutinous matter often covers the micropyle-area (Fig. 129, g), and 

 may extend as an envelope over the greater part, or even the whole of the egg. 



In the egg itself there is usually a separation of a superficial layer 

 consisting of formative protoplasm (Fig. 129, A') from an inner mass 



