EPHEMERIDA, O DON AT A, PLECOPTERA, ET AL. 



195 



invagination of the embryo in this species (Fig. 117) produces a pit in 

 the yolk mass, one wall of which is the embryo itself (gb) ; the other wall on 

 the dorsal side of the egg, the amnion {am). Before the head of Calopteryx 

 withdraws completely, segmentation begins, and appendages appear 

 (Fig. USA). As the embryo lengthens, the caudal end doubles on itself, 

 forming a hook (Fig. 117(7). The tail folds under the abdomen (Fig. 

 USA) and maintains this position even after the embryo withdraws from 

 the yolk. When the axis of the head is finally shifted, the opening behind 

 it is closed by the serosa (Fig. 1185). The amnion in the head region 

 now forms a large sac, part of which lies in contact with the serosa. Just 



am 



A 



Fig. 118. — Calopteryx. Longitudinal sections. A-C, successive stages, (am) Amnion. 

 (am. cav) Amniotic cavity, {am. ser) Fusion area of amnion and serosa, (emb) Embryo. 

 {hi) Head, (ser) Serosa. {From Brandt.) 



before rotation of the embryo, the amnion and serosa fuse where they are 

 in contact, a split appears here, and the envelopes immediately begin to 

 contract. The opening enlarges, but the amnion and serosa remain fused 

 around the edges. The head is forced through the opening as the enve- 

 lopes contract, turns sharply, and moves along the ventral side toward the 

 cephaUc end (Fig. 118C). The embryo continues to grow and takes up 

 most of the ventral side of the egg, while the shrinking envelopes cover the 

 yolk on the dorsal side. The serosa forms the thickened dorsal organ; 

 the amnion, the provisional dorsal closure. 



In Plathemis the gastrula furrow appears on the ventral side while the 

 posterior half of the embryo is immersed. Cells along the midventral side 

 push inward to form the gastrula ridge (Fig. 119 A) which later flattens 



