INSECT 'S STOMACH SNODGRASS 



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in insects. With a closed blastopore the apparent gastrulation pro- 

 duces mesoderm (C, Msd) externally beneath the ectoderm, and 

 endoderm {End) internally, but most of the endoderm is at once 

 disintegrated to scatter its cells in the yolk as viteUophags (D, Vph), 

 which lose no time in beginning their business of digestion. There 

 is finally left only a remnant of the endoderm in the form of a layer 

 of dormant cells resting on top of the mesoderm (D). Thus we see 

 that the insect embryo has violated no fundamental law of develop- 

 ment, and that its deviations from the conventions are necessary 



Figure 11.— Diagrams interpreting the formation of the inner germ layers in insects 



as modified gastrulation. 



A, primitive gastrulation and mesoderm formation. B, the blastopore closing. C, the 

 blastopore closed, the mesoderm covered by the ectoderm, the endodermal stomach 

 {End) in process of disintegration. D, most of the endoderm cells scattered in the yolce 

 as viteUophags (Vph), leaving a ventral remnant (End) resting on the mesoderm. 

 E, lengthwise section of the embryo showing the three remnants of the endoderm (AMR, 

 I MR, PMR) that will later regenerate the stomach. 



adaptations to proper development within an egg filled with yolk. 



The viteUophags are an emergency crew; they have only tempo- 

 rary employment, since a dispersed stomach could not function as 

 a digestive organ for food coming in from the outside. The young 

 insect is destined to live a free life; it must eventually, therefore, 

 have a stomach of the usual kind provided at least with an intake 

 orifice for receiving food material. Hence, it is quite necessary that 

 the embryo sliould soon begin the building of a conventional gastric 

 organ in order to meet its destiny of hatching and being left to 

 its own resources in the outside world. 



It is claimed by some investigators that in some of the arthropods, 

 including certain insects, the wall of the definitive stomach is formed 



