INVOLUTION OF THE EMBRYONIC ENVELOPES. 



307 



embryo. The dorsal tube develops in an altogether similar way in the Orthop- 

 tera. When the enteron at a later stage closes dorsally, the dorsal tube, 

 together with all the food-yolk, is enclosed within it (Fig. 150 F). In Hydro- 

 phihis, the dorsal plate and the dorsal tube are distinguished by their great 

 length (Fig. 149) ; they extend over the whole dorsal surface of the egg. The 

 closing of the dorsal tube, brought about by the fusion of the amniotic folds 

 over it, here takes place from behind forward, so that for some time a pore 

 is found anteriorly as the opening of the dorsal tube (Fig. 149 C). 



B. Involution accompanied by dorsal withdrawal of the 



Amnion only. 

 This type has been observed in a few Coleoptera (Chrysomelidae, 

 Fig. 151). The serosa (s) here remains entirely unaffected by the 

 whole process of involution, and is retained till the late stages of 

 development closely applied to the inner side of the chorion. The 

 provisional enclosure of the yolk dorsally is brought about after 

 the rupture of the ventral amnion by the dorsal growth (Fig. 151 B) 



Fio. 151. — Diagrammatic transverse sections illustrating the formation of the dorsal body-wall 

 in Doryphora (after Wheeler), am, amnion (in B, serving as the provisional dorsal integu- 

 ment ; in C, undergoing disintegration) ; k, germ-band ; s, serosa. 



of the latter (am). "When, in later stages of development, the germ- 

 band extends more and more over the dorsal surface of the egg 

 (Fig, 151 G), the cells of the compressed amnion first accumulate 

 dorsally (this accumulation has been described by Wheeler in 

 Doryphora as the amniotic dorsal organ), and then break away 

 from the aggregate and become scattered in the food-yolk, where 

 they finally disintegrate (Wheeler, No. 95). To this type belong 

 Doryphora (Wheeler), Lina (Graber), and perhaps also Donacia 

 (Melnikoff, No. 53). 



C. Involution accompanied by dorsal withdrawal of the 

 Serosa and complete separation of the Amnion. 

 This type is closely related to the first. It was observed by 

 Graber in Chironomus (Fig. 152) and the Phryganeidae. Here only 



