82 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



ently has made a similar suggestion for Sciara coprophila. More work on 

 late embryonic and early larval stages is necessary for convincing proof 

 of the origin of this membrane in Diptera. 



As has been described in the foregoing chapter, the origin of the 

 mid-gut epithelium has been the source of much controversy. What- 

 ever may be its origin, among the Hymenoptera and Diptera, as well as 

 some Coleoptera, Lepidoptera, etc., at each extremity of the inner 

 (lower) layer, either united with it or contiguous to it, a cell mass, or 



mge 



proct 



mgs 



Fig. 44. — Sagittal section of embryo. Development of mid-gut epithelium, {am) 

 Amnion, {br) Brain, (ggl) Ganglion, {mge and mgs) Mid-gut epithelium, (proct) 

 Proctodaeum. (stom) Stomodaeum. 



mesenteron rudiment (mge), is developed, which by many is regarded as 

 an entoderm derivative. From each of these two cell masses or rudi- 

 ments two ribbons (rb) of cells grow out, those from the anterior mass 

 growing posteriorly, those from the posterior mass growing anteriorly, 

 until their tips meet (Figs. 43, 44). After fusion of their tips the ribbons 

 (rb) widen until they wholly enclose the yolk. The ribbons usually arise 

 from the ventrolateral angles. In some cases, only a single middorsal 

 ribbon arises from each cell mass; in others instead of a ribbon each 

 rudiment develops in the form of a cup, one covering each end of the 

 yolk, the rims of the cups facing each other and finally fusing. In some 

 insects, in addition to the two ribbons, cells liberated from the middle 

 strand of the inner layer contribute toward building the mid-gut. 



