262 INSECT A. 



In this type of development "\ve must also include one of the 

 Gryllidae, Oecanthus, although by so doing we place this form in 

 opposition to the other Orthoptera. In this genus the first rudiment 

 of the germ-band, indeed, does not arise by invagination as has 

 been shown by Ayers (No. 1), but forms as a short ventral plate 

 which is overgrown by the amniotic fold. The inner layer of the 

 fold (the amnion), as in the cephalic fold of the Aphidae, does not 

 at first grow over it as rapidly as the outer layer. The serosa 

 therefore at first forms the only complete covering of the germ- 

 band. The amnion, however, grows out later under the serosa 

 and becomes closed, so that the embryo is finally covered by a 

 double cellular envelope. The germ-band is therefore in this case 

 to be classed among those which are grown over by a fold; and 

 it is also superficial. It lies originally (and this is the point which 

 has determined us in the above classification) on the dorsal side 

 of the egg, with its cephalic end directed posteriorly (Fig. 140 A), 

 and thus in position entirely agrees with Gorixa (p. 279). It is 

 therefore obliged, when the rent has taken place in the embryonic 

 envelopes, to undergo a process of rotation (Fig. 140 B, C, D) so 

 as to attain its definitive position. This process and the later 

 degeneration of the serosa through the formation of an invagination 

 (dorsal tube) show such complete agreement with the processes in 

 other examples of this type that we feel justified in classing 

 Oecanthus among them. 



E. Insects in which the Germ-band is overgrown by the 



Amniotic Fold. 

 Orthoptera genuina. In all the forms belonging to this group 

 as yet investigated, with the exception of Oecanthus, the germ-band 

 lies from the first on the ventral side of the egg, with its cephalic 

 end pointing anteriorly. In these therefore there is no rotation 

 of the germ -band. The embryonic envelopes arise through the 

 formation of folds. The germ-band is in most cases comparatively 

 short (Blatta, Cholodkowskv, No. 19, and Wheeler, No. 95; 

 Stenobothrus and Mantis, Graber, Nos. 26 and 30). Only in 

 Gryllotalpa (Korotneff, No. 47) does the germ-band attain a con- 

 siderable length, and consequently appears with its anterior and 

 posterior ends bent over towards the dorsal side. In all these 

 forms the posterior (abdominal) end of the germ-band in later 

 stages appears flexed ventrally, as is also the case in the Libelhdidae, 

 Rhynchota, Oecanthus, Phryganeidae, in many Coleoptera, and to a 



