No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 79 



been studied. Musca may perhaps be regarded as representing 

 a distinct type, since in this highly modified form the rudi- 

 mental amnion and the serosa are neither shed nor agglomer- 

 ated and engulfed in the yolk, but are supposed to form the 

 definitive body-wall. (Kowalewsky, '86; Graber, '89.) 



It is clear that the revolution of the insect embryo includes 

 three distinct processes : first, the eversion and katatrepsis of 

 the germ-band ; second, the formation of the dorsal walls ; and 

 third, the elimination of the envelopes. The mechanical cause 

 of eversion and katatrepsis is probably a contraction on the part 

 of the envelopes after their fusion and rupture over the ventral 

 surface of the embryo. After the embryo is everted from the 

 amniotic cavity, or exposed after the rupture of the amnion and 

 serosa, these envelopes temporarily form the dorsal covering of 

 the yolk. Do they ever form the definitive dorsal body-wall } 

 For both envelopes this is claimed to be the case only in Musca. 

 In all other insects the serosa, at least, takes no part in forming 

 the permanent body-wall, as it is either shed or engulfed in the 

 yolk. The question is, therefore, restricted to the fate of the 

 amnion. In many insects (Lepidoptera, Hymenoptera Phyto- 

 phaga, some Diptera and Coleoptera), it has been shown that 

 the amnion takes no part in the formation of the definitive body- 

 wall, although a decision on this point is rendered difficult by 

 the fact that no hard and fast line can be drawn between the 

 ectoderm of the germ-band and the cells of the amnion. In 

 other insects the decision is even more difficult. Still, I may 

 say that I have seen nothing in the insects I have studied, 

 to convince me that the amnion is converted into a portion of 

 the permanent body-wall. Even in Musca it seems probable 

 that the amnion and serosa only temporarily function as the 

 body-wall, and that their cells are ultimately replaced by true 

 ectodermal elements from the germ-band. In Blatta and 

 XipJiidiuin I have seen appearances which lead me to believe 

 that at least a part of the amnion may be eliminated by such a 

 process of cell-substitution. I incline, therefore, to the views of 

 Korschelt and Heider ('92), who hold that the envelopes are 

 probably completely eliminated, and that the entire body-wall 

 is derived from the ectoderm of the germ-band. 



