FROM EGG TO INSECT SNODGRASS 405 



have to do with one another in the last chapter. Since this account 

 is not written as a mystery story, it is necessary to mention some 

 thinjis before we come to them, and go back continually and begin 

 over again Avith each set of organs. 



The alimentary canal has been developing along with the appen- 

 dages, the nervous system, the body wall, and other parts not yet 

 described. Its rudiments appear very early, soon after the mesoderm 

 is formed, as two masses of cells that grow inward, one at each end of 

 the latter (fig. 20A, AME, PMR). These cell masses probably be- 

 long to the same layer of cells as the mesoderm — i. e., they and the 

 mesoderm together may constitute the second or inner embrj^onic 

 layer. This is clear in the embryos of animals that have a simpler 

 form of development than insects. Anyhow, the two cell masses are 

 destined to form the stomach of the insect, and for this reason 

 they should be the endoderm. They are called the anterior and the 

 posterior viesenteron imdijnents {AMR and PMR), since embryolo- 

 gists prefer to call the insect stomach the " mesenteron," w hich is 

 scientific Greek for " mid intestine." As the stomach rudiments 

 grow, the first backward, the other forward in the interior of the 

 embryo (fig. 19 B), they take the form of cups in some species and of 

 long bands in others, the edges of which overlap the yolk (P), and 

 come together all around the latter (C), and finally completely in- 

 close it (D). The yolk thus comes to be contained in a closed sac 

 of endodermal tissue, and this sac is the stomach (D, Vent)^ usually 

 known in the adult insect as the ventricuhis, which is scientific Latin 

 for " little stomach." From now on the yolk, which is the food of the 

 embryo, must be absorbed through the walls of the stomach in order 

 to reach the other growing tissues. 



The mature insect, however, needs more than a stomach for ali- 

 mentary purposes. Since it must acquire its food from without, there 

 must be an intake opening from the exterior at one end, and like- 

 wise an opening through Avhich waste parts of the food may be 

 ejected. At the time the stomach rudiments are developing, or in 

 some species before they appear, there is formed a hollow, tubular 

 ingrowth of the ectoderm at the base of each, which elongates inward 

 with the stomach rudiments attached to its inner end (fig. 20 B, C). 

 The anterior ingrowth is known as the stomodeum {Stom), the 

 posterior one as the proctodeum (Proc). Before the insect hatches, 

 usually the inner ends of the stomodeum and the proctodeum open 

 into tiie corresponding ends of the stomach (Z>), and the insect in 

 this manner comes to acquire a continuous alimentary canal open 

 at each end to the exterior, the stomodeum becoming the gullet, or 

 oesophagus {(E), and the protodeum the intestine {Int). With some 

 76041—26 27 



