The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . layered germband thus becomingtransformed irtto aband of two cell-layers(fig. 15). In somecases the inner layeris formed not by in-vagination but byproliferation or by de- lamination. The From Nussbaum in MuUl and Dennys. C«*rM^».,outer of these two Reeved Co. Iv,^,1^I^1h^;,^^ ^ Fi°- I5-—Diagram showing Formation of^l^rlt:i^.i::^^r ^:^%l^?^:^ E,ectodlrm; M, innerlayer {endoblast of »>• Magnified. some authors, fig. 15, M) much difference of opinion has pre-vailed. It has usually been regard
The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . layered germband thus becomingtransformed irtto aband of two cell-layers(fig. 15). In somecases the inner layeris formed not by in-vagination but byproliferation or by de- lamination. The From Nussbaum in MuUl and Dennys. C«*rM^».,outer of these two Reeved Co. Iv,^,1^I^1h^;,^^ ^ Fi°- I5-—Diagram showing Formation of^l^rlt:i^.i::^^r ^:^%l^?^:^ E,ectodlrm; M, innerlayer {endoblast of »>• Magnified. some authors, fig. 15, M) much difference of opinion has pre-vailed. It has usually been regarded as representing both endodermand mesoderm, and the groove which usually leads to its forma-tion has been compared to the abnormally elongated blastoporeof a typical gastrula. No doubt can be entertained that the greaterpart of the inner layer corresponds to the mesoderm of more ordinaryembryos, for the coelomic pouches, the germ-cells, the musculatureand the vascular system all arise from it. Further, there is generalagreement that the chitin-lined fore-gut and hind-gut, which form. 424 HEXAPODA lEMBRYOLOGY the greater part of the digestive tract, arise from ectodermal invagina-tions (stoniodaeum and proctodaeum respectively) at the positionsof the future mouth and anus. The origin of the mid-gut (mesenteron),that has no chitinous lining in the developed insect, is the disputedpoint. According to the classical researches of A. Kowalevsky(1871 and 1887) on the embryology of the water-beetle Hydrophilusand of the muscid flies, an anterior and a posterior endoderm-rudiment both derived from the endoblast become apparentat an early stage, in close association with the stomodaeum andthe proctodaeum respectively. These two endodemi-rudimentsultimately grow together and give rise to the epithelium of the mid-gut. These results were confirmed by the observations of K. Heiderand \V. M. Wheeler (1889) on the embrjos of two beetles—Hy^fro-phitus and Doryphora respectively. V
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