TRANSITION FORMS BETWEEN THE TWO TVrEs. 287 



among the Insecta, but one which apparently also occurs in the Lepidoptera. 

 The dorsal umbilical passage is in this way more and more circumscribed, until, 

 by the fusing of these inner folds and the absorption or rupture of this solid 

 cord, it is completely obliterated (Fig. 143 B). The embryo, whose dorsal wall 

 is now completely formed, lies henceforth entirely free within two cellular sacs, 

 the outer one corresponding to the serosa aud the inner to the amnion (s, am 

 in Fig. 143 B). 



Although the presence of a double cellular envelope (amnion and serosa) in 

 the Hymenoptera can hardly, according to Graber's recent observations, be 

 doubted, we must here mention that other authors expressly point out that 

 only a single embryonic envelope is present, which then must be assumed to 

 be the serosa. Although some confusion on this point may arise from the 

 fact that the inner envelope (amnion) becomes closely applied to the germ- 

 band (Graber) and is indistinguishable from the latter, we cannot deny 

 the possibility that the true amnion at first remains stationary, as was 

 described above (p. 2S1), in the case of the cephalic fold of the Aphidae and 

 Oecanthus (p. 282). There would then be a separation of the amnion from 

 the serosa at the edge of the amniotic fold, and the latter would grow out 

 independently by a process of overgrowth (cf. the description given of the 

 formation of the amnion in the Scorpions, p. 5, Fig. 3). The same conditions 

 were found in Apis by BtJTSCHLl (No. 11) and Grassi (No. 32), also in Pollstes 

 gallica and Chalicodoma muraria by Carriers (No. 13).* 



We are still altogether in doubt as to the presence and constitution of tin- 

 embryonic envelopes in the Pteromalina (cf. on this point the account of 

 Platygaster), in which the endoparasitic life of the embryo and larva has essen- 

 tially modified the course of the development. 



F. Transition forms between the two types of development 



of the Germ-band. 



Coleoptera. The germ-band of the Coleoptera, which, like that 

 of the Hymenoptera, does not attain to any great length, show's in 

 its anterior and principal portion (Fig. 144, Jc) the characters of a 

 germ-band grown over by the embryonic envelopes. It is superficial 

 and is grown over by the forward extension of a caudal fold (at") and 

 paired cephalic folds (af) (Fig. 134 C, of", p. 270), which soon fuse, 

 to which are added, in Lina (Graber, No. 30), lateral folds that arise 

 independently. The posterior end of the germ-band, on the contrary, 

 develops entirely according to the invaginating type described in 

 connection with the Libellulidae. In Hydrophilus (Kowalevsky, 

 No. 48, and Heider, No. 3S), at the posterior end of the rudiment 

 of the germ-band, there is a pit (Fig. 134 A, g, p. 270) which exactly 

 corresponds to the invagination known by authors as the germ- 

 prominence (p. 276). As this pit deepens, the most posterior end 



* [By"KGER (No. II.) has published a full account of the embryology of this 

 bee, based upon CarriErb's notes. He finds only one envelope arising from 

 the peripheral portion of the blastoderm aud persisting for a short time. — Ei>.] 



