366 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The entoderm that will form the epithelial lining of the mid-gut 

 will develop from two masses of cells, which form the two ends of the 

 inner layer. These are not connected by a middle entodermal strand. 

 The remainder of the inner layer lying between the entodermal masses 

 will become the mesoderm. This is similar to the development of the 

 entoderm and mesoderm in Musca, as described by Escherich (1900) and 

 Graber (1889). 



Segmentation begins at the twenty-fifth hour. This process starts 

 first at the cephalic end, the head segments forming and deepening long 



ect 



Fig. 321. — Sciara. Longitudinal 

 section of embryo at 34 hours, {am) 

 Amnion, (ect) Ectoderm, (gc) Germ 

 cells. {U) Inner layer, (y) Yolk. 



Fig. 322. — Sciara. Cross section of coiled 

 embryo at 34 hours (amnion omitted), (ect) 

 Ectoderm, (gc) Germ cells, (il) Inner 

 layer, (y) Yolk. 



before the tail shows any signs of metameric formation. At the anterior 

 pole one of the segmental folds continues to deepen and soon can be 

 identified as the stomodaeum. At the thirty-sixth hour, mandibular, 

 maxillary, and labial segments can be distinguished, with the stomodaeum 

 between the mandibles appearing much deeper than the segmental 

 grooves. 



At the time that segmentation begins, a second longitudinal depres- 

 sion, the neural groove, forms on the ventral side. A ridge of cells pushes 

 inward against the lower layer, separating it into two parts except for the 

 two entodermal cell masses at the ends which are left intact. These 

 terminal masses are the mesenteron rudiments which will later form the 

 mid-gut. 



