OLIGONEPHRIDIA 



265 



daeum, and that the gut suspensorium alone is derived from the secondary 

 entoderm. 



The account and figures given by Hirschler of the development of the 

 embryonic envelopes are not so clear as those given by Will (1888) for 

 Aphis pelargonii. In A. pelargonii, at the place where the serosa joins 

 the amnion (Fig. 188-B) and where the serosa joins the head lobe, the 

 amniotic fold (Fig. 188D, amf) will appear. This fold is two-walled at 

 the base (Fig. 188£',a), but beyond this point the amnion fuses with the 

 serosa into a single membrane which continues around the head lobe to 

 form what Will has designated as the "cephalic-serosa" (Figs. 1S8E; 189, 

 ser. ceph). The structure thus differs in that only the serosa covers the 

 head instead of both amnion and serosa as in many insects. 



The other later processes of development — the rupture of the cephalic 

 embiyonic envelope, the evagination of the embryo through the opening 



(bid) Blasto- 

 Ovarian yolk 



(mycetom). (yc) Yolk cells. 



Fig. 191. — Toxoptera. {bid) Blasto- 

 derm. ((/) Primary yolk, {y 2) Ova- 

 rian yolk (mycetom). {yc) Yolk cells. 



thus produced, and the rotation of the germ band — occur in the same way 

 as described for the Libellulidae. 



The Egg of the Oviparous Sexual Generation. — The modes of develop- 

 ment described above apply to the viviparous parthenogenetic summer 

 generations of aphids. Since the development of the oviparous sexual 

 winter generation differs in some particulars, a brief abstract of the 

 account of Webster and Philhps (1912) of the embryology of the stem 

 mother of the grain aphid {Toxoptera graminum) will serve as an example. 



The eggs are broadly elhpsoidal, with a slight reniform tendency. At 

 oviposition they are pale yellow, changing in a few hours to a faint 

 greenish color, some days later turning black. The chorion is tough and 

 leathery, with a smooth and shining surface. A vitelline membrane is 

 present. At the posterior end is the large, dense, almost spherical granu- 

 lar mycetom (termed by Webster and Philhps "ovarian yolk ") (Fig. 190). 

 The blastoderm forms more rapidly at the anterior than at the posterior 

 pole. Some of the cleavage nuclei remain in the yolk to form yolk cells, 



