PHYLUM ARTHROPODA 249 



of the cells of the germ band and the other of the blastoderm cells. 

 The amniotic cavity is then cut off from the exterior. The 

 invagination which has brought the germ band to lie within the 

 amniotic cavity has seriously altered the orientation of the germ 

 band with reference to other parts of the egg. The ventral 

 surface of the embryo, which originated on the ventral external 

 surface of the egg, has by invagination come to be directed 

 toward the dorsal side of the egg. While in this position (C and 

 D), the embryonic appendages make their appearance. Soon 

 afterward the embryo is everted from the amniotic cavity and 

 again comes to lie (E) on the surface of the egg with its parts 

 coinciding with the original orientation of the egg. 



In the embryological development of most of the Orthoptera, 

 Trichoptera, Diptera, Lepidoptera, and Hymenoptera the over- 

 grown or superficial type of germ band occurs. The invaginated 

 or immersed type of germ band is found in some of the Odonata, 

 Coleoptera, Thysanoptera, and Hemiptera. Transitional con- 

 ditions, which seem to be intermediate between these two types, 

 are found in some of the Coleoptera and the Orthoptera. 



Internal Metamorphosis and the Imaginal Discs. — In both 

 types of metamorphosis, an insect undergoes radical changes in 

 its internal organization before reaching the adult stage. Even 

 organs which are found ahke in the larval and adult stages do not 

 pass over directly from one stage to the next, but through the 

 processes of histolysis the tissues of one stage disappear and 

 entirely new tissues are formed through the processes known as 

 histogenesis. In the Holometabola, where entirely new struc- 

 tures such as wings and legs become functional for the first time 

 with attainment of the adult form, still more profound internal 

 changes accompany metamorphosis. During the larval and 

 pupal stages, rudiments of the legs, wings, and head appendages 

 make their appearance as internal buds. These imaginal discs, 

 as they are called, are, throughout their early development, 

 enclosed within internal sacs (Fig. 126 A). The hypoderm of the 

 body wall invaginates to form these sacs, and the rudiment of the 

 appendage which each of these sacs contains lies thus within a 

 cavity entirely surrounded by hypoderm. Only in later develop- 

 ment (Fig. 126 C and D) do the sacs open and allow the develop- 

 ing appendages to extend freely beyond the body surface. 

 The development and transformation of the imaginal discs of 

 the head and thorax of a fly are shown in Fig. 126. 



