FKOM EGG TO INSECT — SNODGRASS 393 



In still other species, the cells along the middle part of the germ 

 band, which part may be slightly grooved (F), multiply by hori- 

 zontal division and give off cells from their inner ends which 

 constitute the inner layer (F, Meso). The first method of forming 

 the inner layer is probably to be regarded as the typical oue, and 

 the others as modifications; it is to be noted that the modifications, 

 as is usually the case in developmental processes, are but short-cuts 

 to the same end as that attained more laboriousl}^ b3^ the oiubnary 

 method of procedure. 



By one or another of these methods the insect embryo comes to be 

 changed from a one-layered to a two-layered condition. In this it 

 follows the rule of all other animal embryos, though all do not arrive 

 at the two-layered stage in the same way. Those embryos that con- 

 sist of a hollow ball of cells in the blastoderm stage become two- 

 layered simply by an infolding of one wall until they have the form 

 of a double-walled cup. This method is regarded as typical of 

 animals in gen ral, and such processes as those that occur in insects 

 are presumed to be derived from it in some way. With most embryos, 

 furthermore, the next step consists in the formation of a third layer, 

 derived from the second, which comes to lie between the other two. 

 The three embryonic layers are then known as the ectoderm (the 

 outer layer), the mesodenn (the middle layer), and the endoderm 

 (the inner layer). These cell layers constitute three separate sets of 

 embryonic building material, each of which will develop into sj)ecific 

 tissues of the adult, and no one of which will normally substitute for 

 another. Tln' ectoderm, for example, always forms the outermost 

 covering of the body and all parts derived from it; the endoderm 

 forms the alimentary canal, or the essential parts of it ; and the meso- 

 derm forms the tissues between, including those of the muscks, the 

 fat, the heart, the internal reproductive organs. Organs and sys- 

 tems of organs, however, often result from a combination of tissues 

 produced by two or by all thiee of the embryonic layers. In de- 

 scriptions, therefore, it is better to follow th? development of organs 

 rather than to trace separately the tissues that are produced from 

 each embryonic layer. 



Insect embryos do not closely observe the general rule for the 

 formation of the embryonic layers. In the two-layered stage, the 

 outer layer is the ectoderm (fig. 13 C, E, F, Ect)^ but the inner 

 layer, as its subsequent history shows, is mostly the mesoderm 

 {Meso). The endodermal tissues arise from ingrowths at the two 

 ends of the mesoderm, as will later be described in tracing the forma- 

 tion of the alimentary canal. 



Development proceeds by different rates of growth in the dif- 

 ferent parts of the germ band and the tissues resulting from it, 

 shapes being attained by gradual infoldings or outfoldings, with 



