302 INSECTA. 



E. Transition to the Definitive Form of Body. 



The development of the definitive shape of the body is accomplished 

 through the circumcrescence of the Avhole of the nutritive yolk by 

 the germ-band. We have seen above (p. 272) that, in the later 

 stages of development, the germ-band as a rule lies in such a way 

 that its anterior end corresponds to the anterior pole of the egg, 

 and its posterior end to the posterior pole. As the germ-band grows 

 considerably in breadth, its lateral edges shift up dorsally over the 

 surface of the food-yolk (Figs. 150 A-F, 169, 170, 171, and 172). 

 In this way the lateral parts, and later the dorsal parts, of the larval 

 body are formed. By means of this circumcrescence, the food-yolk 

 comes to lie entirely within the embryo, and finally fills the lumen 

 of the archenteron (Fig. 150 F). The closing of the larval body 

 dorsally through the circumcrescence of the food-yolk by the germ- 

 band is so intimately connected with the degeneration of the 

 embryonic envelopes that we shall have to return to these processes 

 later on. 



The dorsal parts of the embryo in the cephalic region develop 

 independently of the broadening of the germ-band described above. 

 The segments of this region, i.e., the maxillary, only take part to 

 a small extent in the development of the dorsal portion, the latter 

 being mainly formed by the bending over dorsally and the backward 

 extension of the cephalic lobes as well as of the procephalon. The 

 anterior end of the germ-band is therefore here bent over dorsally. 

 An actual dorsal flexure of the cephalic region develops, as was first 

 pointed out by Weismann and later by Hatschek and H eider (No. 

 38). During this flexure of the anterior end of the body, the part, 

 of the procephalon lying near the mouth appears as a transverse 

 swelling (labrum). The former most anterior part of the procephalon 

 now becomes the clypeus and assumes a more backward position. 

 The cephalic lobes in this process of shifting pass towards the 

 dorsal side, and the antennal rudiments consequently shift in front 

 of or above the mouth. 



5. Completion of the dorsal part of the Embryo and 

 degeneration of the Embryonic Envelopes. 



Tn most of the Arthropoda that have so far come under review 

 (Crustacea, Arachnida, Myriopoda, etc.), development takes place 

 through the formation of a so-called germ-band, but without the 

 formation of actual embryonic envelopes. The surface of the whole 



