274 INSECTA. 



The germ-band is superficial in many Orthoptera (Oecanthus, Gryllotalpa, 

 Blatta, Mantis), in many Hemiptera (Corixa), in the Phryganeidae, the Diptera, 

 and the Hymenoptera. In the Coleoptera also, the greater part of the germ- 

 band appears superficial, but its posterior end is in the first stages immersed. 

 An immersed germ-band occurs in the Libellulidae, many Hemiptera (Pyrrho- 

 coris), many Orthoptera (Stenobothrus), and in the Lepidoptera. 



C. The distinction between the invaginated Germ-band and the 

 Germ-band that has been overgrown by the Membranes.* 



With regard to the manner in which the germ-band arises and 

 to its position, there are two opposite types among the Insecta, 

 these, however, being connected by means of transitional forms. 

 In the one type the ventral plate is invaginated into the inner 

 part of the egg, and in the other the amniotic folds rising from 

 its edges grow over it. 



1. When the germ-band during its formation is invaginated, e.g., 

 Libellulidae, Brandt (No. 7), its first rudiment appears in the form 

 of a small thickening of the blastoderm lying ventrally in the posterior 

 half of the egg (ventral plate, Figs. 136 A, bp and 137 ^4), in the 

 posterior region of which invagination soon takes place (Fig. 136 A, 

 Jch). The lumen of this invagination is the first rudiment of the 

 amniotic cavity (Fig. 1 36 B, ah) ; the thickened ventral portion of 

 its wall (k) forms the germ-band, while the thinner dorsal portion 

 gives rise to the amnion (Fig. 136 B, C, am). The blind end of the 

 invagination denotes the later anal end of the germ-band (&'). Since, 

 however, the invagination grows from behind forward in the egg, it 

 results that the primitive position of the germ-band appears to be the 

 exact reverse of its later position, its posterior end becoming forwardly 

 directed, while its cephalic end lies near the posterior pole of the 

 egg. In a similar way, that surface of the germ-band which was 

 primarily on the ventral surface of the egg becomes secondarily turned 

 towards the dorsal side of the egg, so that the ventral surface of the 

 developing embryo is now seen through the dorsal wall of the egg. 

 To bring the germ -band into its definitive position the process 

 described as reversal, rotation, eversion, or revolution, which will 

 be described below, is needed. 



It should be mentioned that in eggs of this type the anterior end of the germ- 

 band, which is distinguished by the extension of the cephalic lobes (Fig. 136 



* On this distinction rests the division of Insects into those with inner and 

 those with outer germ-bands (inner and outer germs, or entoblastic and ecto- 

 blastic forms, Grabek). Gkaber has recently suggested the terms entoptic 

 and ectoptic germ-formation to describe these categories. We have not adopted 

 these terms, because they are liable to confusion with the superficial and 

 immersed germ-bands, given above (p. 273). 



