196 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



out as the inner layer. The area of fusion between the amnion and 

 serosa in Plathemis extends along the anterior part of the embryo. When 

 the spUt appears, the head does not push through; but the envelopes, as 

 they contract, pull back from the ventral side to expose the embryo. As 



Fig. 119. 



A 



-Plathemis. Cross sections. 

 (ggv) Gastrula furrow 



A, 43-hour; B, 70-hour embryo, (am) Amnion. 

 (il) Inner layer, (ser) Serosa. 



this occurs, the head moves toward the cephalic pole, and the tail with- 

 draws from the yolk. The head lobes now extend over on the dorsal side 

 (Fig. 120 A). The serosa, as it contracts, thickens and finally lies as a 

 strip on the dorsal side, covering the remainder of the dorsum to form the 

 provisional dorsal closure. The serosa soon sinks into the yolk where it 



^neurp 

 • br 



stem 



proci 



A B C 



Fig. 120. — Plathemis. Sagittal sections. A-C, successive stages, (.am) Amnion. 

 (br) Brain, (do) Secondary dorsal organ, (nc) Nerve cord, {neurp) Neuropile. (prod) 

 Proctodaeum. (ser) Serosa, (siom) Stomodaeum. {y) Yolk. 



forms the secondary dorsal organ, an ovoid hollow mass of cells apparently 

 attached to the end of the stomodaeum (Fig. l20B,do). 



The stomodaeum which probably forms at the time segmentation 

 takes place in the invaginated embryo is well developed when the embryo 



