OLIGONEPHRIDIA 



273 



The head lobes at this time appear in cross section as two thickened 

 areas continuous with the serosa {ser), forming the covering enclosing 

 the yolk (Fig. 205C,hl). In a section closer to the pit through which the 

 embryo is pushing into the yolk (Fig. 205B) the two furrows representing 

 the cephalic end of the amniotic cavity are evident {am. cav). Still farther 

 cephalad the relation that the head lobes ihl) bear to the germ band {gh) 

 and the amnion {am) is apparent in 

 Fig. 205A . The mass of cells separated 

 from the two amniotic furrows repre- 

 sents that point in the embryonic 

 envelopes where the amnion and serosa 

 join {am. ser). 



Before the head lobes have with- 

 drawn from the surface at about 32 

 hours, the gastrular groove appears, 

 and the inner layer forms from the 

 invaginating ridge (Fig. 204). The 

 formation of the inner layer is uneven. 

 At one point in the middle region it 

 appears flattened out with the groove 

 already obliterated beneath it (Fig. 

 2065, t7); at other points it appears as 

 invaginating masses of cells (Figs. 205C, 

 206 A, D); and at the anterior and pos- 

 terior ends it appears as a definite ridge 

 with a groove in the ectoderm. Figure 

 206 C shows the inner layer consisting 

 of only three cells with the groove in the 

 ectoderm entirely obliterated. The 

 inner layer forms in a similar manner 

 in Pyrrhocoris. According to Seidel 

 these alternate thicker and thinner 

 masses of cells of the inner layer repre- 

 sent an indication of early segmenta- 

 tion. The number of segments that 

 develop in the embryo as indicated by the masses of cells is 20, of which 6 

 are head segments, 3 thoracic segments, and 11 abdominal segments. 



The Heteroptera differ from most other orders in the position assumed 

 in the egg by the embryo when it invaginates into the yolk. It is clear 

 from Fig. 204 that the ventral side of the germ band is directed inwardly 

 and that when appendages appear they extend into the yolk, the inner 

 layer and developing mesenteron ribbons (Fig. 208, mge) lying on the 

 outer side of the germ band near to the surface of the egg. This position 



Fig. 204. — Oncopeltus. Sagittal 

 section, (aw) Amnion, (ect) Ecto- 

 derm, (hi) Head lobe, {il) Inner 

 layer, (ser) Serosa. 



