44 WHEELER. [Vol. VI 1 1. 



The view is now pretty generally held that in the Insecta 

 both mesoderm and entoderm arise from a median longitudinal 

 furrow — the former layer throughout nearly the entire length, 

 the latter only in the oral and anal regions of the germ-band — 

 and that the vitellophags, or cells left in the yolk at a time 

 when the remaining cleavage products are traveling to the sur- 

 face to form the blastoderm, take no part whatsoever in the 

 formation of the mesenteron, but degenerate in situ and 

 finally undergo dissolution. Discussions of the literature on 

 this subject are to be found in the papers of Heider ('89) and 

 Graber ('89, '90), and so few are the facts accumulated since 

 these resumes were penned that I may dispense with an 

 historical consideration of the insect germ-layers in the present 

 paper. 



In the interpretation of the insect gastrula the entoderm has 

 always played an important role. The origin of the mesoderm 

 has long been known and has been duly provided for in the 

 various germ-layer hypotheses which have from time to time 

 been advanced. But the true origin of the lining of the mid-gut 

 has been ascertained only within the last few years, so that we 

 cannot expect to find an adequate treatment of this layer in 

 the older theories. Led astray by what had been observed in 

 Crustacea and Arachnida, some writers chose to regard the 

 vitellophags as forming the mesenteron and shaped their 

 theories accordingly (Oscar and Richard Hertwig, '8l). But 

 now that it has been shown that the vitellophags take no 

 part in forming the lining of the mid-gut, their morphological 

 position is rendered even more obscure, and we are brought 

 face to face with the question : Are the vitellophags a portion 

 of the entoderm which has been set apart very early in develop- 

 ment for the purpose of yolk-liquefaction or are they an entirely 

 new segregation of cells belonging to none of the conventional 

 germ-layers .-* Those who defend the former alternative main- 

 tain that the vitellophags of insects are entodermal in function 

 inasmuch as they digest yolk and closely resemble the amoeboid 

 Crustacean yolk-cells which afe actually converted into the lin- 

 ing of the mesenteron. On the other hand it is urged, that as 

 the yolk-cells arise and function before the blastoderm is com- 



