76 THE EMBRYOLOGY OF THE HONEY BEE 



evsky in 1886 published a brief paper on the origin of the iru 

 teron rudiments in Musca. In this insect the mesenteron 

 rudiments were formed, much as Grassi had described them in 

 Apis, from the anterior and posterior ends of the middle plate 

 (mesoderm). Kowalevsky constructed an ingenious theory to 

 account for the origin of the germ layers in insects, comparing 

 the conditions which he found in the insect embryo with those 

 occurring in a marine invertebrate, Sagitta. He regarded the 

 insect egg, at the time of the formation of the germ layers, as 

 comparable to a gastrula so much stretched out or elongated 

 that the entoderm (mesenteron rudiments) was pulled into two 

 halves. The views of Grassi (see above p. 70) and Kowalevsky 

 were later accepted by Wheeler (1889), Cholodkowsky (1891), 

 and by Heider in Korschelt and Heider's textbook, and re- 

 cently in a modified form, by Nusbaum and his pupils (II. 2). 



These results did not however find universal acceptance, since 

 Witlaczil, Voeltzkow and Graber, like Ganin, contended that the 

 mesenteron arose from the blind inner ends of the proctodaeum 

 and stomodaeum. In 1895 Heymons, in his handsome mono- 

 graph on the development of the Orthoptera and Dermaptera, 

 devoted especial attention to the development of the mesenteron, 

 and also found that it was derived from the blind inner ends of 

 the stomodaeal and proctodaeal imaginations. This work has 

 had a wide influence and Heymons' results have been confirmed 

 by a number of investigators in the Coleoptera, Lepidoptera and 

 Orthoptera. 



Heymons recognized more fully than his predecessors the 

 theoretical difficulty involved in deriving the mesenteron from 

 ectoderm, since identification of the mesenteron of insects with 

 entoderm is thereby precluded. Heymons boldly met the diffi- 

 culty by supposing that the functional mesenteron of pterygote 

 insects is of comparatively recent origin, and that the original 

 entoderm is now represented by the yolk cells, which therefore 

 may be considered as constituting a vestigial or degenerate 

 mesenteron. This view received support by Heymons' discovery 

 (1897) tna t m Lepisma, a primitive apterygote insect, the func- 

 tional mesenteron is actually formed from yolk cells and by 

 Madame Tschuproff-Heymons' discovery (1903) that in the 

 Odonata the mesenteron is formed in part by yolk cells and in 



