NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS IQ 



vitellophags (yolk eaters), they probably serve for digestion of yolk 

 during early stages of development; they should therefore be con- 

 sidered as endodermal. In some of the lower insects the yolk cells 

 are said to form a stomach wall by investing the yolk. In such cases 

 the yolk cells seem to demonstrate their endodermal nature. 



With most of the higher insects a narrow ventral strip of the 

 blastoderm becomes differentiated from the lateral plates, and either 

 sinks into the yolk or is overgrown by the lateral plates, A ventral 

 groove is thus formed along the ventral side of the blastoderm ; this 

 ventral groove has been regarded as a remnant of the elongated 

 blastopore closed between the mouth anteriorly and the anus pos- 

 teriorly. The enclosed ventral plate spreads out and divides into 

 an inner endodermal layer and an outer mesodermal layer. 



The early embryo of Onychophora presents a median ventral 

 groove that eventually closes between the two ends which become 

 the mouth and the anus. The onychophoran, therefore, has long 

 been thought to give an example of the primitive blastopore of the 

 arthropods. Manton (1949), however, has shown that this mouth- 

 anus groove of the onychophoran is not the blastopore. The endo- 

 derm, she says, is proliferated from an area behind the anus, and 

 cells from this area form the complete stomach epithelium. The 

 mouth-anus groove is thus a secondary formation, though perhaps in 

 some way derived from the blastopore. 



Even in the insects it must be noted that the concavity of the 

 ventral plate does not become the stomach lumen. The functional 

 endoderm is proliferated from cell masses at the two ends of the 

 endoderm, and in some cases also from the whole length of it. The 

 growth of the definitive midgut epithelium from cell masses thus 

 resembles the proliferation of endoderm in the Onychophora and 

 of yolk cells in the lower insects. The business of endoderm is to 

 surround the yolk in order to digest it. This is accomplished mostly 

 by the anterior and posterior cell masses which send out ribbons or 

 sheets of cells toward each other around the yolk; these eventually 

 unite and form the stomach which thus comes to contain most or all 

 of the remaining food (yolk) of the developing embryo. 



Since in some cases the endoderm appears to be proliferated from 

 the inner ends of the stomodaeum and proctodaeum, some embryolo- 

 gists have contended that the insect stomach is ectodermal. It is 

 noted by Henson (1946), however, that the two ends of the stomach 

 represent the extremities of the blastopore where naturally ectoderm 

 should be generated externally and endoderm internally. It is evi- 



