20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



dent, then, that while ancestral recapitulation plays no part in the 

 formation of the insect's stomach, the embryo has adopted another 

 method of gastrulation, namely cell proliferation, and thus does not 

 violate the germ layer theory. 



Mesenteron: As indicated by the origin of the word (Gr. meso, 

 middle, + enteron, alimentary canal) this is the middle portion of 

 the food tract. It extends from the ectodermal stomodaeum in front 

 to the ectodermal proctodaeum behind, and it becomes the functional 

 stomach of the insect, known as the ventriculus. It is probably always 

 of endodermal origin though variously formed in the embyro (see 

 Gastrulation; also Snodgrass, 1935, and DuPorte, 1960). 



In form the mesenteron is typically an elongate cylindrical sac, but 

 it may be a slender coiled tube, and in some insects it is differenti- 

 ated by constrictions into several well-defined sections. The anterior 

 part that surrounds the stomodaeal funnel is called the cardia. Blind 

 tubular pouches, known as gastric caecae and varying in length and 

 number, project from various parts of the stomach wall. Most 

 commonly, however, they project from the anterior end of the 

 stomach around the entrance of the stomodaeum. 



The wall of the mesenteron is a thick epithelium of columnar cells 

 separated from the hemocoel by a distinct basement membrane. Ex- 

 ternally, beyond the basement membrane, there is a muscular sheath 

 of longitudinally and circularly arranged fibers, but the arrange- 

 ment differs from that around the stomodaeum and proctodaeum in 

 that the longitudinal fibers are external to the circular ones. The 

 inner ends of the epithelial cells are somewhat irregular and, as 

 seen in sections, present what is known as a striated border due to the 

 presence of alternating dark and clear lines that give a brushlike 

 appearance. All the epithelial cells probably function for both secre- 

 tion and absorption. Simple secretions are discharged through the 

 striated border, but the cells also go through a disruptive process 

 that has commonly been regarded as a form of holocrine secretion. 

 Globules filled with granular material are extruded into the stomach 

 lumen ; these are then constricted and break off, followed by dissolu- 

 tion of their walls and the scattering of their contents. Whether 

 this is a process of secretion discharge or simply a degeneration 

 and dissolution of the cells, or both, it results in such a destruction 

 of cells that they must be continuously replaced. Replacement is 

 effected by groups of regenerative cells intercalated between the 

 bases of the epithelial cells; by mitotic division these regenerative 

 cells give rise to new cells that replace the worn out or discarded 



