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TEXT-BOOK OF ENTOMOLOGY 



section. In the second type only the amnion, in the third only the serosa are 

 concerned in this rupture, while in the fourth type both membranes remain 

 A intact until the slipping out of the larva. 



(Korschelt and Heider.) 



j. Formation of the germ-layers 



The older views on the structure of 

 the layers of the primitive band of in- 

 sects were thoroughly unsatisfactory. 

 Biitschli first found that in the bee, by 

 a kind of folding process, an inner 

 layer of the primitive band arose. 

 Soon afterwards Kowalevsky, by the 

 employment of section-cutting and 

 thorough researches, laid the founda- 

 tion of a more exact knowledge of 

 these layers. He found that in 

 Hydrophilus a furrow extended along 

 the whole length of the primitive 

 band (Fig. 515, A, B, r), which, while 

 invaginating or sinking in, gave rise 

 to the inner layer of the primitive 

 band, i.e. the common rudiment of 

 endoderm and mesoderm (Fig. 539, 

 A-C). 



Kowalevsky also found similar con- 

 ditions in the honey-bee (Apis), 

 Lepidoptera, and other forms. The 

 furrow above mentioned must be re- 

 garded as a very long gastrula invag- 

 ination, extending along the entire 

 ventral side of the embryo, and the 

 edges of the furrow as a long-drawn- 

 out blastopore. The tube arising in 

 Hydrophilus through the closing of 

 the furrow we may regard as a primi- 

 tive intestinal canal. 



The first rudiment of the gastrula 

 furrow appears in insects as two folds 

 extending along both sides of the 

 median line in the thickened ventral 

 plate (Fig. 536, /), through whose 



FIG. 534. Diagram showing the for- 

 mation of the embryonic membranes in 

 Lepidoptera (A, after Kowalevsky, B and 

 C, after Tichomirotf) : k, primitive band ; 

 am, amnion; /*, serosa; do, yolk; rrl, 

 invagination of the fore-gut, ed, of the 

 hind-gut ; ra, mouth ; an, anus ; x, dor- 

 sal umbilical passage. From Korschelt 

 and Heider. 



