y8 WHEELER. [Vol. VIII. 



4. TJie Eliviinatioii of the Embryonic Envelopes, 



Anatrcpsis and katatrepsis in the lower insect orders, or the 

 completion of the envelopes and their rupture in the higher 

 orders, are separated by a distinct interval, during which the 

 germ-band undergoes a considerable development. *But during 

 this interval, the diapause, no change is noticeable in the 

 envelopes themselves beyond a thinning of the amnion with 

 the increased growth of the embryo. The elimination of the 

 envelopes is preceded by katatrepsis just as their formation 

 was preceded or accompanied by anatrepsis. This elimination 

 is immediately followed by the completion of the dorsal body- 

 wall and may take place in a variety of ways. Korschelt and 

 Heider ('92) distinguish the following types in this process: 



1. The amnion and serosa become continuous and, after the 

 eversion of the embryo, are drawn back over the yolk to form 

 a single layer of cells. As the dorsad growth of the body-walls 

 proceeds, both envelopes are drawn together and pushed into 

 the yolk to form a sack or longitudinal tube which is ultimately 

 enclosed by the walls of the mesenteron and absorbed. To 

 this type belong the Odonata, Rhynchota, some Orthoptera 

 {Blatta, QicantJms, Gryllotalpd) and some Coleoptera {Hy- 

 dropJiilus) . 



2. The serosa is shed from the yolk and the amnion alone 

 contracts on the dorsal surface preparatory to being drawn into 

 the yolk and absorbed. (Certain Coleoptera, e.g. DorypJiora) 



3. The serosa alone is agglomerated and drawn into the 

 dorsal yolk, the amnion being cast off. (Certain Diptera 

 \CJiironomus\ and Trichoptera.) 



4. Both envelopes are shed. (Lepidoptera and certain 

 Hymenoptera.) 



In Xiphidhmi we may perhaps recognize a fifth type, in 

 which as in the fourth, both amnion and serosa are shed. But 

 while the serosa is in great part shed as a simple membrane, 

 the indusium which is a modified portion of the serosa, together 

 with the amnion is drawn together in a mass and cut off from 

 the embryo. It is more than probable that other types of 

 envelope elimination will be discovered when more forms have 



