380 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 193 3 



by a reassembling of the scattered vitellophags on the outside of the 

 undigested remnant of the yolk. There is nothing improbable about 

 this method of reestablishing a disintegrated stomach, but its actual 

 occurrence is perhaps not fully substantiated, and some embryologists 

 claim that if a stomach is thus formed it is a transient structure. 

 There is, on the other hand, no doubt that the final stomach of most 

 arthropods is a structure regenerated from the temporarily dormant 

 remnant or remnants of the endoderm left at the two ends of the 

 mesoderm, or forming a median strand of endodermal tissue along 

 the length of the mesoderm (fig. 11 E). These regenerative endoder- 

 mal remnants, since they form the definitive stomach, which is 

 termed the mesenteron, are known as the anterior mesenteron rudi- 

 ment (AMR), the posterior mesenteron rudiment (PMR), and the 

 intermediate mesenteron rudiment {I MR). 



The final process of the stomach regeneration is fairly simple and 

 direct. The several mesenteron rudiments (fig. 12 A) begin a pe- 

 riod of active growth. Their cells multiply and spread outward from 

 their margins around the yolk. The anterior and posterior rudi- 

 ments form cup-shaped extensions that approach each other from 

 the opposite ends of the body (B, C), the intermediate rudiment, if 

 present, contributes to the space between them, until finally the 

 yolk is completely invested in a covering consisting of a single layer 

 of endodermal cells (D). The embryo now has acquired its defini- 

 tive stomach, or mesenteron, which proceeds to digest the yolk 

 within it, and later takes care of the food that enters by way of the 

 mouth. The vitellophags appear to be out of a job. Some investi- 

 gators claim that they perish by degeneration, and are absorbed. 

 The blood of adult insects, however, contains free cells that are sus- 

 piciously like the vitellophags; they devour and digest loose matter 

 and possibly disease germs in the blood, and thus resemble the white 

 blood corpuscles of vertebrate animals. These insect blood cells are 

 known as phagocytes (i.e., eating cells). Perhaps they are descend- 

 ants of the wandering vitellophags. 



The mesenteron of the embryo is yet a closed sac. A stomach 

 without an inlet or an outlet would be a useless luxury for a free- 

 living animal, but it is entirely appropriate to an embryo confined 

 within an egg shell. During the process of the stomach formation 

 two pockets of the ectoderm grow inward at opposite ends of the 

 body (fig. 12 B, C, D), the first carrying the anterior mesenteron 

 rudiment, the second the posterior mesenteron rudiment. When the 

 mesenteron is completely formed (D), therefore, it is suspended 

 between two ectodermal tubes which are open to the exterior but 

 closed at their stomach ends. Shortly before the insect hatches, 

 however, the partitions between these tubes and the stomach sac 



