INSECTS IN WHICH THE GERM-BAND IS OVERGROWN. 



i'sr, 



rudiment of the band or ventral 

 plate is only a round, shield-like 

 thickening of the blastoderm (Fig. 

 142 A), from the edge of which 

 the amniotic fold rises. Only later 

 does the germ - band begin to 

 lengthen, and very soon, by the 

 passage of food -yolk masses be- 

 tween the amnion and the serosa, 

 becomes immersed (Fig. 142 B). 

 Since, as in the Diptera, increasing 

 length leads to the sharp dorsal 

 curvature of the germ-band, and 

 since the amniotic cavity follows 

 this curvature, that dorsal portion 

 which represents the connection 

 between the embryo and the germ- 

 bands appears to become more and 

 more limited (Fig. 142 C, at x). 

 There thus develops a dorsal um- 

 bilical passage which is here of 

 significance in so far as it repre- 

 sents the passage through which 

 the food-yolk mass taken into the 

 interior of the embryo communi- 

 cates with that lying between the 

 amnion and the serosa. Taking 

 into account this feature, it might 

 be said that, in the Lepidoptera, 

 the embryo is surrounded by a 

 yolk-sac connected with it through 

 the dorsal umbilical passage. 



Hymenoptera. In the Hymen- 

 optera, conditions are found which 

 agree in essentials with those 

 described for the Diptera. The 

 germ -band here also is always 

 superficial, and is covered by a 

 double cellular envelope (amnion 

 and serosa) formed by the growth 

 ventralwards of a cephalic and a 



am 



a/7i 



Fig. 142. — Diagram of the formation of the 

 embryonic envelopes in the Lepidoptera 

 (A after Kowalevsky, B ami C after 

 Tichomiroff). k, germ-band ; am, am- 

 nion ; se, serosa; do, food -yolk; vd, 

 invagination of the stomodaeum ; ed, 

 invagination of the proctodaeum ; m, 

 mouth ; an, anal aperture ; x, dorsal 

 umbilical passage. 



