270 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



cavity closes. The embryo now lies on the dorsal side of the amniotic 

 cavity (Figs. 199, 200) with its caudal extremity toward the anterior and 

 its cephalic extremity toward the posterior end of the egg, its ventral 

 surface turned toward the ventral aspect of the egg. In this position the 

 embiyo develops until about the fifth day, when revolution takes place. 

 On the second day, before the amniotic cavity closes, an "indusial thick- 

 ening" appears on the anterodorsal area of the blastoderm, similar to that 

 described by Wheeler in XipMdium (Fig. 36, ind). Muir and Kershaw 

 did not follow the development of the indusium in detail, but they state 

 that later two indusial envelopes develop, so 

 it may be assumed that these are formed from 

 the two walls of the indusial sac as in Xiphidium 

 (Figs. 36, 37, ind). The outer indusial enve- 

 lope (Figs. 199, 200, 201, ind. 1) becomes closely 

 applied to the serosa, except at the two poles, 

 and remains intact until the hatching of the 

 nymph; the inner indusial envelope (Figs. 199, 

 200, ind. 2) joins the amnion near the head of 

 the embryo and takes on the usual functions of 

 the serosa during the revolution of the embryo. 

 Between the fifth and sixth days the amnion 

 and the inner indusial envelope break open 

 near the head of the embryo, and the revolu- 

 tion begins. The head of the embryo leaves 

 the amniotic cavity and moves upward to the 

 dorsal side of the egg, toward the original posi- 

 tion of the germ band. When the head is well 

 on the dorsal side, the entire embryo, including 

 the amnion and the inner indusial envelope, 

 begins to revolve around the longitudinal axis 

 of the egg. The emergence from the amniotic 

 cavity continues at the same time (Fig. 200) until the embryo lies on 

 the ventral side of the egg in the normal position of insects during the 

 later stages of embryonic development. 



An embryonic cuticle covers the embryo, entirely sheathing all append- 

 ages. A thickening at the cephalic portions of this cuticle forms the 

 "egg burster." Thus the mature embryo is enveloped in three coverings, 

 apart from the chorion; viz., the serosa (Fig. 201, ser), forming the outer 

 covering; the outer indusial envelope (Fig. 201, ind.l.)] and the "embry- 

 onic cuticle" (Fig. 201, cut). Although this was not seen by Muir and 

 Kershaw, it seems possible that the amnion and the inner indusial 

 envelope, after rupturing, are absorbed in the yolk and thus undergo the 

 same fate as the amnion and serosa of the Odonata. 



post 



Fig. 201. — Siphanta. 

 eral aspect at 12 days. 

 Chorion, (cut) Cuticula. 

 Dorsal side of egg. 



Lat- 



ich) 



idr) 



(emb) 



Embryo, {ind 1) Outer in- 

 dusium. {mi) Micropylar 

 area, {ser) Serosa. 



