i62 Embryonic Development of CJiironomus 



Origin of 

 inner 

 germinal 

 layers. 



applies to tlie earlier stage, before the ectoderm reunites 

 beneath the infolded cells. 



In most of the insects whose development has been 

 carefully studied ^ the entoderm and the mesoderm are 

 developed in the following way : — the infolded cells 

 along the greater part of the length of the fold become 

 mesoderm- cells, bat at the oral and anal ends of the line 

 median cell-masses form, which are the rudiments of the 



-4 A- 







'i^-..*/- 



\% 







•^ 



Fig. iji — Transverse sections of embryo, showing successive 

 stages of the develiiping niesenteron. v.j>^ ventral plate. 

 y)i\ prominences from which the ento-mesoderm is derived. 

 Ih, lateral bands of ento-mesoderm. n, nerve-cord. (From 

 Eitter, 1890, figs. 30-33.) 



entoderm. These are pushed into the interior by the oral 

 and anal invaginations (fore-gut and hind-gut). A pair 

 of cellular strings then grow backwards from the anterior 

 cell-mass, and a similar pair forwards from the posterior 

 mass. The two pairs approach, meet on either side, 



' e. g. Musca ^Kowale^vsky), Apis (Grass!'), Hj-dropliilus (Heider), 

 Doi-yphora (Wheeler', and Chalicodoma (Carriere). 



