INSECTS IN WHICH THE GERM-BAND IS OVERGROWN. 



283 



still greater degree in the Lepidoptera and certain Hymenoptera. 

 This curvature becomes lost and the abdomen straightens as a rule 

 even before hatching. 



It should be mentioned that in Stenobothrus the amniotic fold forms at a 

 very early period in the development of the germ-band. At a time when 

 gastrulation is commencing and the ventral plate is still round and shield-like, 

 the latter is already grown over by the amniotic fold (Graber, No. 26). In 

 GriillotaJpa, the embryonic envelopes develop in the form of two folds rising 

 laterally. In Blatta, a caudal fold and paired cephalic folds corresponding 

 to the two cephalic lobes appear first (as in the Coleoptera). The germ-band 

 in the Orthoptera is, as a rule, superficial. Only in Stenobothrus does it become 

 immersed by the appearance of particles of food-yolk between the two enveloping 

 layers, a condition which we shall meet with again in the Lepidoptera. 



Fig. 141. — Lateral aspect of the egg of Chironomus in the stage at which the embryonic 

 envelopes develop (diagram adapted from Weismann and Kupffer). A, showing the 

 commencement of the cephalic and caudal folds (kf and .«/). B, union of the two folds 

 laterally and their continuous advance over the germ-band, v, ventral side ; d, dorsal side ; 

 ft, uncovered portion of the germ-band ; am, amnion ; do, food-yolk ; k) cephalic end of 

 the germ-band ; k', ventral portion of the germ-band ; k", portion of the germ-band bent 

 over dorsally ; k"', hook-like bend of the posterior end ; kf, cephalic fold of the amnion ; 

 kl, cephalic lobe ; r, dorsal umbilical passage ; s, serosa ; sf, caudal fold of the amnion 

 x, x', union of amnion with the ectoderm. 



Diptera. In all Diptera as yet examined, the germ-band attains 

 a considerable length ; it thus not only covers the ventral side, but 

 its two ends bend over dorsally, the posterior end extending very 

 far forward (Fig. 141), so that on the dorsal side of the egg they 



