NEUROPTERA AND COLEOPTERA 



291 



cells, which arise later from the tip of the stomodaeal invagination and 

 which will surround a portion of the yolk somewhat as in the Odonata, 

 may then be designated as the secondary entoderm. 



The embryo elongates and at the same time decreases in diameter in 

 the dorsoventral direction (Fig. 225). A lower layer develops diffusely 

 without the formation of a gastrula furrow (Fig. 226); the head (antr) 

 and tail end (post) are at this time differentiated, the former less thickened 

 than the latter. The amnion-like envelope becomes progressively 

 thinner; the yolk, owing apparently to pressure, becomes longer and 

 thinner. At this stage one observes in the amniotic cavity small struc- 

 tureless masses which stain differently from adjacent cells (Fig. 226, par). 



•oos\ 



anir 



Fig. 227. — Siylops nycthemerae. (am) 

 Amnion, {antr) Anterior, (post) Poste- 

 rior end. (vent) Venter, (y) Yolk. 



anir 



•posi 



.am 



Fig. 228. — Stylops. {am) Amnion. 

 {antr) Anterior end. {i. am) Inner and 

 (o. am) outer section of amnion, {par) 

 Paracyte. {post) Posterior end. 



These are interpreted by Noskiewiez and Poluszynski as either symbionts 

 or, more probably, paracytes. 



Further proliferation of the cells of the embryo produces an ingrowth 

 of the caudal end whereby apparently a pressure is exerted on the yolk. 

 In Fig. 227 the yolk is shown partly divided, a small one- or two-nucleated 

 portion at the center, a larger portion at the surface (Fig. 227, y). 



In the development of viviparous or parasitic insects Noskiewiez and 

 Poluszynski (1927) say that if the yolk has preserved its primary ento- 

 dermal potentiality, it will not degenerate before it has served its function 

 in relation to the development of the mid-gut epithelium, even though it 

 may have been considerably reduced in size. If, however, the potential- 

 ity has been lost, one of two things may happen: either it degenerates 

 quickly and completely, or it will be used in the formation of a secondary 

 envelope. For example, in Stylops and Halictoxenus a small part of the 

 yolk aids in the formation of the mid-gut epithelium; on the other hand, 

 the larger part of the yolk in the former insect degenerates ; in the latter it 

 contributes to the formation of the trophamnion-like envelope. 



