INSECTS WITH INYAGINATED GERM-BAND. 



281 



amniotic cavity, which originally belonged to the posterior pole, in the course 

 -of further development shifts more to the dorsal side. At the same time, the 

 rudiments of the cephalic lobes (k), which have arisen as blastodermic thicken- 

 ings and which formerly lay at the anterior pole of the egg, shift backward over 

 the ventral side so that finally they extend over the posterior pole (Fig. 139 D). 

 The whole of this blastodermic thickening is not, as in the Libellulidae, 

 included in the invagination of the germ-band, and its true anterior end is 

 therefore not at first covered by the embryonic envelopes. Soon, however, 

 a circular amniotic fold appears surrounding the primitive anterior end of the 

 germ-band and the aperture of invagination (Fig. 139 D, of). This circular 

 fold at the time of its rise consists, like every other amnion-fold, of two layers 

 (amnion and serosa). In the course of further growth, however, the serosa 

 outstrips the amnion, so that the cephalic lobes appear covered by only one 

 epithelial cell-layer, the so-called cephalic serosa (Fig. 139 E, s').* 



d~* 



v 



Fig. 140. — Rotation of the embryo of Oecanthus (diagrams after Ayers). a, anterior pole 

 of the egg ; am, amnion ; b, posterior pole of the egg ; d, dorsal side of the egg ; k, germ- 

 band ; r, dorsal plate (caused by the contraction of the serosa); s, serosa; v, ventral side 



The other, later processes of development — the rupture of the cephalic 

 embryonic envelope, the evagination of the embryo through the aperture 

 thus produced, and the rotation of the germ-band occur in just the same way 

 as in the Libellulidae, 



The ontogeny of the Aphidae has been described chiefly by Brandt (No. 7), 

 Metschnikoff (No. 55), Witlaczil (No. 98), and Will (No. 97). In the 

 above account we have principally followed "Will. 



* This is a case of the imperfect development of the amnion, such as has 

 been asserted for certain Hymenoptera. It should be mentioned that the account 

 given by Brandt makes it appear possible that the embryonic envelopes in the 

 cephalic region in the Coccidac (and perhaps even also in the Libellulidae) 

 develop in the way described by Will in connection with the Aphidae. In 

 this case, in these forms also, the envelope covering the cephalic region would 

 consist of a single layer of cells. 



