EMBRYONIC ENVELOPES, DORSAL ORGANS, BLASTOKINESIS 53 



toward the center of the egg. Simultaneously the proamniotic zone (pra) 

 is stretched and occupies the posterior half of the egg (Fig. 34) with a 

 corresponding reduction in the size of the proserosa {prs). The egg of 

 Machilis being laterally compressed, the furrow that carries the embryo 

 inward is not in the nature of an invagination, since it is open at the sides, 

 exposing the lateral margins of the embryo. The furrow is not precisely 

 comparable to an amniotic cavity, for its walls are made up of a part of 

 both proserosa and proamnion, for which reason Heymons chose the 

 terms ''proserosa" and "proamnion" rather than " serosa " and " amnion " 

 for the membranes in this group of insects. 



In Lepisma saccharina the form of the envelopes shows a further 

 advance toward the development of an amnion and serosa. Here 

 the minute embryonic disk has sunk into the yolk, carrying with it the 

 amniotic epithelium which joins the periphery of the disk with the 

 serosa. The germ band forms the bottom of a sac (Fig. 109) whose sides 



emb 



Fig. 34. — Machilis alternata. {emb) Embryo, {.pro) Proamnion, {prs) Proserosa. 

 {From Heymons.) 



constitute the amnion. The sac remains open, communicating with the 

 exterior by the amniotic pore. Were this pore to close, we should have 

 the normal pterygote structure. When the second stage of blastokinesis 

 occurs, the sac turns inside out, without the necessity of the fusion of the 

 envelopes at the head end or of their rupture (Fig. 111). When blasto- 

 kinesis is completed, the amnion, serosa, and embryo form the walls of a 

 closed yolk sac as in the Pterygota. In the process the amnion becomes 

 extended, with a corresponding shrinkage of the serosa (Fig. 111). 



Turning now to the Pterygota, we shall find that in the most generally 

 encountered types both amnion and serosa are developed and that the 

 other more specialized types may in most cases be readily derived from 

 them. 



The following synopsis of the Eutracheata will indicate the variations 

 that occur. Too great a rehance should, of course, not be placed on a 

 tabulation of this sort, since the developmental history of the members 

 of too few families among the orders are known to admit of broad generali- 



