No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 15 



responding process already completed in the embryo. From 

 among the numerous preparations which I have made I select 

 for illustration one (Fig. 18) which seems to show the process 

 clearly. In surface view the organ would appear as in Fig. 3, 

 The spreading of the serosal cells over the edges of the 

 disk from all sides is now seen to be due to a process of 

 induplication, or folding. The circular fold is, of course, 

 cut in two places in the median transverse section figured. 

 It advances in such a manner as to leave the outer face of 

 the indusium evenly rounded and undisturbed, the upper sur- 

 face of the fold usually forming a continuous line both with 

 the outer surface of the serosa and with the median still 

 uncovered portion of the organ. The fold continues to advance 

 from all sides till the layers of which it consists meet and 

 become confluent in essentially the same manner as the folds 

 that form the amniotic and serosal layers over the embryo 

 proper. We now have three layers of cells. (Fig. 19.) The 

 outermost layer, s, is the serosa which has everywhere the same 

 structure and evenly envelops the whole Qgg, having been 

 separated first from the embryo and now by a similar process 

 also from the indusium (Fig. II, B). The innermost layer 

 consists of the unchanged greater portion of the organ. The 

 median layer, to judge from its component cells, seems to 

 be derived exclusively from cells of the original body of the 

 organ and not from the serosa. This layer is, therefore, like 

 the amnion of the embryo proper, structurally more closely 

 related to the body it envelops than to the serosa. Fig. 18 

 favors this conclusion, which presupposes that only the outer 

 half of the circular fold is derived from the serosa, for in this 

 section the lower and thicker layer of the fold on either side 

 certainly consists of cells derived from the body of the organ. 

 Even before the layers are fully formed the edges of the two- 

 layered organ are sharp and somewhat irregular (Fig. 18), not 

 rounded like the edges of the embryo when its amnion is com- 

 pleted. The whole organ still has essentially the same form 

 that it had in the stage represented in Fig. 17. 



It will be convenient to name the different layers of cells, 

 thus far distinguished. For the amnion of the embryo proper 



