278 



IXSECTA. 



occupied by the serosa lias become considerably diminished (Fig. 

 138 C). This envelope now contracts towards the anterior pole of 

 the egg, thickening considerably at the same time (Fig. 138 C, se). 

 In consequence of this contraction, the edge of the rent where the 

 serosa and the amnion coalesced and finally the amnion itself are 

 drawn anteriorly over the food-yolk (Fig. 138 C, se and am), so 



that the two envelopes 

 together finally form 

 a sac lying dorsally 

 to the embryo ; this 

 sac is filled with food- 

 yolk, and may be 

 defined as a kind of 

 (dorsal) yolk-sac. As 

 the lateral and dorsal 

 parts of the embryo 

 now develop further, 

 the contents of the 

 yolk-sac are taken up 

 more and more into 

 the intestinal cavity, 

 which communicates 

 with the sac, and are 

 used up, so that finally, 

 by a process to be 

 described later, the 

 serosa itself is (appar- 

 ently) drawn into the 

 embryo and assimilated 

 (p. 304). 



Fig. 13S. — Three stages in the development of Caloy- 

 (after Brandt, from Balfour's Text-book). The embryo 

 is represented inside the egg-shell, a, secondary opening 

 of the amniotic cavity, through which the embryo 

 emerges ; ab, abdomen ; am, amnion ; at, antenna : md, 

 mandible ; to.?; 1 , m./-, first and second maxillae ; oc, oeso- 

 phagus ; p\ p", j- 3 , the three pairs of thoracic legs ; se, 

 serosa ; v, anterior end of the germ-band. 



Since the germ-band of the Libcllulidae arises by an invagination -which 

 grows into the interior of the egg, it is seen that the amnion and serosa are 

 here separated by a wide space filled with food-yolk. The germ-band of the 

 Libcllulidae is therefore immersed. Its cephalic end, however, is excepted 

 from this immersion, and so belongs to the superficial type. 



Rhynchota. The type of development of the germ-band described 

 above for the Libelhdidae occurs also, as far as is at present known, 

 in all Rhynchota. Metschnikoff (ISTo. 55) and Brandt (Xo. 7) 

 thus found in Hydrometra, and Graber (No. 27) in Pyrrhocoris, 

 conditions of development of the egg which in all important points 

 agreed with those observed in the Libelhdidae. 



