72 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



was represented by the yolk cells which therefore constituted the vestigial 

 mesenteron. Support for this view was found in Lepisma and Odonata 

 where what seem to be yolk cells appear to form functional mesenteron. 

 Followers of this view consider the definitive mid-gut epithelium as an 

 ectoderm derivative and that entoderm derivatives are wholly lacking 

 in postembryonic life. Under this assumption the formation of the 

 so-called "gastral furrow" is a secondary acquisition, since it does not 

 involve an entoderm. 



Eastham (1930a) maintains that there is no essential difference 

 between a mid-gut that develops from rudiments arising directly from 

 the germ band and one that arises from anterior and posterior cell 

 proliferations at the bhnd ends of the stomodaeum and proctodaeum. 

 In the former case cell proliferation to form entoderm begins before the 

 stomodaeal and proctodaeal invaginations, whereas in the latter case 

 entodermal proliferation begins later, after the invaginations have 

 occurred. Heymons (1895a, 1905), who seems to be in agreement so 

 far as the facts are concerned, states that the divergent views of authors 

 are largely a matter of interpretation. However, since he regards the 

 yolk cells as the entoderm, he interprets the mid-gut epithelium as 

 ectoderm. Nusbaum and Fulinski (1909) in summarizing their views 

 list seven different ways in which the mid-gut may develop: 



Type I. Entoderm rudiments are first cut off from the germ band. This is followed 

 by the invagination of stomodaeum and proctodaeum carrying the entoderm into 

 the yolk. Noack (1901) in Muscids; Hirschler (1909) in Donacia. 



Type II. Entoderm rudiments form from the germ band, but the stomodaeum 

 invaginates before these are cut off from the ectoderm. Karawaiew (1893) in 

 Pyrrhocoris. 



Type III. In anterior region as type II. In posterior region as type I. Nusbaum 

 and FuUnski (1909) in Gryllotalpa. 



Type IV. Stomodaeum and proctodaeum develop immediately in front of and 

 behind the entoderm rudiments, respectively. Nusbaum and Fulinski (1906) in 

 Phyllodromia. 



Type V. Stomodaeum, proctodaeum, and entoderm rudiments develop concur- 

 rently, and inner layer contributes to mid-gut formation. Hirschler (1909a) in 

 Gastroidea. 



Type VI. As in type V, but no inner layer contributes to entoderm formation. 

 Carriere and Burger (1898) in Chalicodoma. 



Type VII. Growth areas of entoderm rudiments remain dormant till the stomo- 

 daeum and proctodaeum appear. Heymons (1895) in Forficula. In this type the 

 latent, or dormant, rudiments being indistinguishable from the ectoderm cells of 

 the invaginations have led Heymons and his followers to regard the mid-gut 

 epithelium as an ectoderm derivative. 



Several authors who on theoretical grounds are not willing to admit 

 the ectodermal origin of the mid-gut epithelium in Orthoptera, Dermap- 

 tera, and some other insects where the epithelium appears to be of 



