276 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



ectoderm including the evaginations for the appendages. The coelomic 

 sacs are here open toward the yolk. Later the yolk retracts from the 

 germ band, leaving free an open space, the definitive body cavity as 

 indicated in Fig. 208 of Ariasa tristis. The first differentiation of the inner 

 layer on the margin of the body cavity consists of the segmentation of 

 the two longitudinal strands to form the muscle layers of the alimentary 

 canal and of the body wall and the gonads. The anlage of the muscula- 

 ture of the mid-gut separates from the rest of the inner layer and forms 

 with the corresponding anlagen of the preceding and following segments 



O O 



o o 



Fig. 208. — Anasa tristis. Cross section through thorax, (am) Amnion, {am. cav) 

 Amniotic cavity, (ect) Ectoderm, (entm) Ental membrane, {eps) Epineural sinus. 

 (mge) Mid-gut epithelium, (nc) Nerve cord, (p) Leg. {ser) Serosa, isomm) Somatic 

 mesoderm, {splm) Splanchnic mesoderm. 



on each side, from stomodaeum to proctodaeum, a band (Fig. 208, splm) 

 which will later be lined with the backward- and forward-growing strips 

 of the enteron rudiments (mge). 



In Pyrrhocoris the appendages have penetrated far into the yolk and 

 must be drawn from their amniotic envelopes before blastokinesis takes 

 place (Fig. 2035). The germ band shortens; the amnion and serosa tear 

 at their point of fusion ; and in a very brief period the embryo pushes out 

 of the rent and crowds forward on the ventral side, the head reaching the 

 anterior pole of the egg (Fig. 209). As a result of this change in position 

 the serosa (ser) forms a cap over the head of the embryo and at the 

 termination of blastokinesis is absorbed into the yolk on the dorsal side. 

 At the conclusion of blastokinesis the embryo broadens, and the lateral 



