56 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



in some cases at the posterior end 



. . . . Coleoptera (in part), Hymenoptera {Tenthredinidae) 



VIII. Amnion and serosa fully formed, do not fuse at head end; no well-formed 



secondary dorsal organ; germ band not immersed 



Siphonaptera, Nemocerous Diptera 



IX. Serosa fully developed; amnion rudimentary or lacking 



Aculeate Hymenoptera 



X. Embryonic envelope having a nutritive function. . . .Parasitic Hymenoptera 

 XI. Dorsal wall of the embryo represented by the primary epithelium, the cells 



forming a serosa-like covering; amnion present in rudimentary form 



Diptera {Muscoidea, Melophagus) 



The type represented by group III might have been derived from 

 one resembhng type II. Type III is quite generahzed, its development 

 described in the account of a typical insect given on page 9. From 

 this with slight modifications types IV to IX may be derived. Types X 

 and XI are highly specialized. A few groups given in the analytical 

 table are so modified by reason of some peculiar type of development 

 (parasitic or viviparous) that their relation to the other types is not 

 obvious. 



DORSAL CLOSURE 



Three different types of structures are concerned in closing the embryo 

 on the dorsal side, two of them provisional, the other definitive. Roonwal 

 (1937) states that in the embryo of Locusta migratoria, at about the time 

 the proctodaeum appears, a thin membranous provisional dorsal closure 

 is formed. It arises from the lateral edges of the embryo at a point 

 slightly above the origin of the amnion and covers the entire dorsum. 

 By its formation it cuts off the yolk from contact with the embryonic inner 

 side and encloses, for the first time, an epineural sinus which contains a 

 few blood cells but no yolk particles. It is probably of ectodermal 

 derivation and quite independent of the amnion in origin, although it 

 resembles it in structure. Graber (18886) noticed an analogous structure 

 in Stenohothrus variabilis which arises from the lateral edges of the germ 

 band as two flaps that spread medially toward each other and ultimately 

 fuse into a single membrane. Miller (1939) has described a similar mem- 

 brane which he designates as the "ectal membrane," occurring in the 

 embryo of Pteronarcys proteus, which serves as a base for the spread of 

 the definitive mid-gut epithelial cells. A similar structure is also present 

 in the head louse (Scholzel, 1937). The yolk-cell membrane in Carausius 

 described by Leuzinger (1926), though apparently having a different 

 origin, has a similar function. In Locusta, as described by Roonwal, the 

 portion of the provisional dorsal closure lying between the bhnd ends 

 of the stomodaeal and proctodaeal invaginations snaps at the edges and 

 grows around the yolk. Middorsally it fuses with the amnion which 



