306 



INSECT A. 



Corixa and Hydrometra ; also Metschnikoff and Witlaczil in the Aphidae) 

 and in most Orthoptera genuina (Blatta, Wheeler ; Oecanthus, Ayers, No. 1 ; 

 GryUotalpa, Korotneff, No. 47). 



Among the Coleoptera, in which the posterior end of the germ-band arises 

 by invagination, a few forms belong to the type of transformation just described 

 (e.g., Hydrophilus, Kowalevsky, No. 48, Heider, No. 37, Graber, No. 27; 

 and Melolontha, Graber, No. 27). The only distinction is that here the 

 rupture of the embryonic envelopes takes place only after the completion of 



Fig. 150.— Diagrams illustrating the formation of the dorsal organ in Hydrophilus (after 

 Graber and Kowalevsky, from Lang's Text-book). A, transverse section through the- 

 egg, the germ-band being still covered by the amnion (a) and serosa (s). B, the fused 

 amnion and the serosa are now ruptured and drawn back as two lateral folds. C, con- 

 traction of the serosa (s), which becomes the dorsal plate, leads to the dorsal displacement 

 of these folds (Fig. 140 A). D, the contracted serosa is now partly covered by these folds, 

 which are now bent round dorsally (Fig. 149 jB). E, the dorsal tube is completed by the- 

 fusion of these folds (Fig. 141' C). F, the enteron has become completed dorsally and has- 

 enclosed the dorsal tube (s). a, amnion ; d, food-yolk ; ec, ectoderm ; h, heart ; I, body- 

 cavity ; m, rudiment of enteron ; n, nervous system ; s, serosa (in Caud D = dorsal plate, in 

 E and F= dorsal tube) ; tr, lateral tracheal trunk. 



the rotation (p. 288), at a time when the germ-band already lies ventrally and 

 is superficial. The fused embryonic envelopes rupture in the median line and 

 draw back to the sides of the germ-band, where they form folds exactly like 

 those at the commencement of their development (Fig. 150 B). As these folds 

 bend dorsally over the thickened dorsal plate (s, Fig. 150 D) and fuse in the 

 dorsal median line, a complete tube is formed lined by the serosa (dorsal tube, 

 Fig. 150 E), while the amnion provisionally completes the dorsal part of the 



