No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 7 



cells are at first more closely aggregated. The spaces between 

 these centres are subsequently filled in by tangential cell- 

 divisions. Of such centres I can distinguish four: two of them, 

 the precursors of the procepbalic lobes, are paired, while the 

 other two form respectively the growing caudal end of the 

 ventral plate and what I shall call the indusium.^ The indusial 

 centre, which does not make its appearance till a short time 

 after the other centres are formed, does not join the body 

 of the embryo till after the spaces between the procephalic 

 and caudal centres are filled in. This is distinctly seen in Fig. 

 I (Stage A) where the somewhat T-shaped embryo is already 

 established and distinctly marked off, at least posteriorly, from 

 the undifferentiated blastoderm. The nuclei of the blastoderm 

 are as yet no larger than the nuclei of the ventral plate. 

 Numerous caryokinetic figures in all parts of the embryo bear 

 witness to active cell proliferation. No such figures were to 

 be seen in the extra-embryonal blastoderm during and after 

 this stage. The ventral plate including the indusium is 

 scarcely a fifth as long as the ^%g, being much smaller in pro- 

 portion to the size of the yolk than in some other Orthoptera 

 {Blatta, Gryllotalpd). 



The blastopore is seen in the stage figured as a very narrow 

 but distinct groove extending from the oral region to the 

 caudal end of the embryo, where it bifurcates before its 

 termination. The infolded cells give rise to the mesoderm 

 and also, I believe, to the entoderm. 



In XipJiidiimi the three folds that form the amnion and 

 serosa arise like their homologues in Blatta. The first appears 

 as a crescentic duplication surrounding the caudal end; thence 

 it grows forward and after enveloping the whole postoral 

 portion of the embryo coalesces with the two head-folds, each 

 of which arises from the edge of a procephalic lobe. The pro- 

 gress of the anal fold is shown in Fig. 2 (Stage B) PI. I. 

 Although agreeing in its main features with what has been 

 described for most insect embryos, the process of envelope- 



^In a preliminary note ('9C) this structure was called the praeoral plate 

 (Praoralplatte). Many reasons have led me to abandon this term together with 

 others referring to the parts of the organ in its subsequent development. 



