EMBRYONIC ENVELOPES, DORSAL ORGANS, BLASTOKINESIS 57 



now forms the second provisional dorsal closure of the embryo (Fig. 152). 

 At this stage the first dorsal closure thus forms a temporary mid-gut 

 epithelium. Afterward the splanchnic mesoderm grows around and 

 separates the first dorsal closure from the amnion. The former dorsal clo- 

 sure then degenerates, leaving the inner layer of the splanchnic mesoderm 

 as the second temporary mid-gut epithelium until the definitive mid-gut 

 epithelium is formed. 



The second type of provisional dorsal closure, which takes place at 

 the time of revolution, is formed either by the combined amnion and 

 serosa or by the amnion or the serosa alone. Here the membranes, 

 instead of separating yolk from embryo, form an envelope consisting of 

 the membrane (or membranes) on the dorsal side and the embryo on the 

 ventral and lateral sides to enclose the yolk. Both envelopes are involved 

 in members of the groups III, IV, and V described on page 55 and include 

 many orders, among them the Orthoptera, neuropteroids, Hemiptera, 

 and some Coleoptera. Only a small section of the amnion (Fig. 311) 

 forms the provisional dorsal closure in the members of groups VI and VII 

 which include the Lepidoptera, some Hymenoptera (Tenthredinidae), 

 and some Coleoptera {Brachyrhinus, etc.). In the aculeate Hymenoptera 

 (group IX) a strip of the primary epithelium (Apis) or of the serosa 

 (Formica) forms the provisional dorsal closure, whereas in the chilopod 

 Scolopendra the membrana dorsalis performs this function. 



The third, or definitive, dorsal closure is formed by the ectoderm. 

 In Eutermes, a member of group III, as described by Strindberg (19136), 

 both amnion and serosa after rupture form a dorsal covering, the serosa 

 becoming the secondary dorsal organ. The dorsad growth of the body 

 wall gradually forces these membranes into the yolk, without either of 

 them taking part in the formation of the definitive wall. In Chrysomela, 

 a member of group VII, the provisional dorsal closure consists of a 

 longitudinal strip of the amnion. With the dorsad advance of the edges 

 of the body wall the strip becomes narrower by compression and finally 

 sinks into the yolk, the definitive body wall being formed by the ecto- 

 derm. In Formica, a member of group IX, it is a serosal strip that is 

 crowded into the yolk by the advancing edges of the definitive body wall. 

 In the apterygote genus Isotoma, Phihptschenko likewise found that the 

 primary epithelium which forms a provisional dorsal closure is not trans- 

 formed into the definitive ectoderm but is replaced by the dorsad growing 

 body walls of the insect. 



CUTICULAR ENVELOPES 



After the embryonic envelopes have formed, Slifer (1937) and Cole 

 and Jahn (1937) point out that in various grasshoppers (Melanoplus, 

 Chortophaga, Romalea, and Locusta) two membranes are secreted on the 



