The process of budding as above described may be varied or complicated in various ways; thus a secondary, amnion-like, ectodermal covering or ectotheca (fig.
The origin of the mid-gut (mesenteron), outer layers become continuous, forming respectively the amnion that has no chitinous lining in the developed insect, is the disputed and the serosa (fig.
On this view Wheeler, however, compares with the " dorsal organ " the peculiar the entire food-canal in most Hexapoda must be regarded as of extra embryonic membrane or indusium which he has observed ectodermal origin, the " endoblast " represents mesoderm only, between serosa and amnion in the embryo of the grasshopper and the median furrow whence it arises can be no longer compared Xiphidium.
S, serosa; A, amnion; E, ectoderm; N, rudiment of nerveof the oral piece.
Or " vertex," the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging membrane analogous to the amnion of higher Vertebrates andto the ocular segment), while the mandibular and maxillary segments known by the same term.
Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall.
In some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk.
In moths and certain saw-flies there is no rupture of the membranes; the Russian zoologists Tichomirov and Kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa.
Von Baer in 1828, 5 Muller calls the attention of naturalists to the important fact, that while all the Squamata possess an amnion and an allantois, these structures are absent in the embryos of all the Nuda.