52 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



l■e^'olution, one will note that here also the dorsum is ooA'ered bj' the thin 

 amnion and that in. front of this is the more or less thickened contracted 

 serosa in the form of the secondar}^ dorsal organ (Fig. 24C). The origin, 

 position, and fate of the secondary dorsal organ of insects correspond so 

 closely with that of the cephalic disk of Scolopendra that one can scarcely 

 question that these structures are homologous. The only real difference 

 lies in the fact that the membrana dorsalis of Scolopendra will later form 

 the definitive dorsal closure, whereas in insects the amnion forms only a 

 provisional one. If one accepts the foregoing interpretation of Heymons, 

 then the cephahc disk and the membrana dorsalis of Scolopendra are 

 respectively homologous with the serosa and amnion of the pterygote 

 insects. The position, time of origin, and fate of the dorsal organ of 



A B 



Fig. ' 33. — Machilis alternata. (gh) Germ band, (pro) Proamnion, (prs) Proserosa. 

 {From Heymons.) 



the Diplura (Campodea) and the cephalic disk of Scolopendra so closely 

 resemble each other that Heymons considered them likewise homologous 

 structures. 



To account for the formation of the amnioserosal folds Heymons 

 points to the condition found in Machilis and Lepisma, both thysanurans. 

 The germ band in the laterally compressed egg of M. alternata is small 

 and hes at the posterior pole of the egg (Heymons, 1905), its lateral walls 

 merging into the primary epithelium which covers the egg (Fig. 33). 

 Two zones are to be distinguished in the epithelium: a small-nucleate 

 smaller zone immediately surrounding the germ band (p?'a) and a second 

 much larger zone (prs) with large nuclei. The smaller zone Heymons 

 designated as the ''proamnion," the larger as the "proserosa." In the 

 course of development the germ band sinks into the yolk and is carried 



