284 INSECTA. 



are seen lying somewhat near each other (x, %'). The later develop- 

 ment, in which the posterior end of the germ-band draws back to 

 the posterior pole of the egg, is therefore attended by great shorten- 

 ing of the embryonic rudiment. 



The amniotic fold does not here arise simultaneously along the 

 whole edge of the germ-band, but a fold first rises round the cephalic 

 end (hf) and a second at the posterior end (sf) of the germ-band 

 {cephalic and caudal folds of the amnion). Only later do folds 

 form at the sides of the germ-band and connect the cephalic with 

 the caudal fold (Chironomus, Weismann, No. 87, and Kupffer ; 

 Simulia, Metschnikoff, No. 55). 



As the ends of the germ-band are bent over dorsally and lie very near each 

 other, the edges of the amnion also, which arise from the ectoderm in front 

 of and behind the ends of the germ-band (at x and x' in Fig. 141 A), lie near 

 each other. It follows that the region in which the serosa lies directly on the 

 surface of the food-yolk (>•) is in these forms very limited. Similar conditions 

 will be met with in the Lepidoptera and the Phryganeidae. 



The germ-band in the Diptera is throughout superficial ; only its most 

 posterior end, in Chironomus and Simulia, and possibly also in the Muscidae, 

 appears bent in the shape of a hook and sunk into the yolk (Fig. 141 B, &'"). 

 We have here an approach to conditions to be described in the Coleoptera. 



The fact that in a few Diptera the amniotic folds remain imperfect and 

 never completely grow over the germ-band deserves mention. This is the 

 case, according to Metschnikoff (No. 55), in the embryos of the viviparous 

 Cecidomyia larvae, in which the cephalic and caudal folds appear as rudiments, 

 but do not develop further. It is also the case, according to Kowalevsky 

 and Geabeu (Nos. 27 and 28) in the Muscidae, in which the cephalic fold 

 remains extremely small, and only the caudal fold develops somewhat 

 more distinctly. In the later development of the embryo, these folds simply 

 flatten out again and then take a certain part, as it appears, in the develop- 

 ment of the dorsal integument. 



Trichoptera. The conditions to be observed in the rounded 

 egg of the Phryganeidae, as Patten found in Neophylax (No. 65), 

 approximate very closely to the normal type of the Diptera (Chiro- 

 nomus). The very long, superficial germ-band here also covers the 

 greater part of the periphery of the egg, so that its anterior and 

 posterior ends almost touch dorsally. We shall see that even the 

 phenomena of degeneration of the germ-envelopes in the two groups 

 belong essentially, to the same type (Graber, No. 27). 



Lepidoptera. The Lepidoptera also, in the general conditions of 

 the germ-band and the embryonic envelopes, stand very near the 

 two last groups. A remarkable point in their development is that 

 the amniotic fold forms at a very early stage of the development 

 of the germ-band (as in Stenobothrus, p. 282), at a time when the 



