INSECT 'S STOMACH — SNODGRASS 



381 



are broken down, and now at lust the creature has a continuous ali- 

 mentary canal. Thus by a round-about path of embryonic develop- 

 ment, this important organ has resumed the conventional form and 

 structure that the alimentary canal must have acquired at an early 



AMR 



AT^R 



ProG 

 Msd 



PMR. 



■Pro( 



--An 



Ment 



Figure 12. — Diagrams illustrating the regeneration of the stomach from the eudoderm 

 remnants, and tlic formation of the alimentary canal. 



A, 1< ngthwlse section of the embryo, showing the endoderm remnant.<!, known as the 

 anterior, intermediate, and posterior m(>s('ntcron rudiments (AMR, IMR, PMR), and 

 the beginning of the stomodaeal and proctodacal ingrowths of the ectoderm {Siom, 

 Proo.). B, the endoderm remnants boginiilng to grow around the yolk to form the 

 stomach. C, a more advanced stage in the stomach formation. D, the endodermal 

 stomach, or mesenteron {Alcnt), completed as the middle section of the alimentary 

 canal, the ectodermal stomodacum (i>torn) and proctodaeum (Proc) forming its anterior 

 and posterior parts. 



period in the ancestral history of the free-living progenitors of the 

 insects (fig. 7 13). Throughout the life of the insect the food tract 

 preserves the evidence of its triple origin, since its three parts, the 

 stomodaeum, the mesenteron, and the proctodaeum, are always dis- 



