GROWTH AND TRANSFORMATION 157 



recent work upon this subject the only reliable support 

 afforded to this view is from R. Heymons' research (1897) 

 into the development of the bristle- tail Lepisma, and H. von 

 Tschuproff's account (1903) of the origin of the germ- 

 layers in certain dragon-flies (Fig. 41 , C). In the latter case 

 the central portion, in the former the whole of the mid-gut 

 is said to arise from yolk-cells. Most recent workers in 

 this field state that the mid-gut arises from two rudiments 

 which grow from the inner ends of the fore-gut and hind- 

 gut, respectively backwards and forwards, till they meet ; 

 the origin of these rudiments appears to prove that they 

 are ectotermal. Such is the interpretation given by 

 R. Heymons (1895) of the development of cockroaches and 

 crickets, by A. Lecaillon (1898) and P. Deegener (1900) 

 of that of various beetles (see Fig. 41 , B), and by K. Toyama 

 (1902) of that of the silkworm. But many other investi- 

 gators of insectan embryology have described the mid-gut 

 rudiments as arising from two cell-masses of the inner layer 

 towards the front and hinder ends of the germ-band, so that 

 they can be fairly regarded as endoderm. Such is the 

 interpretation of W. M. Wheeler (1893) from his studies 

 of the germ-layers in cockroaches and beetles, of K. Heider 

 (1889) in his classical research on the water beetle Hydro- 

 philus, of K. Escherich (1900) working at fly embryos, 

 of J. Nusbaum and B. Filinski (1906, 1909) from researches 

 on crickets and cockroaches, of J. Hirschler (1909) from 

 investigations on the development of the beetle Donacia, 

 and of J. A. Nelson (191 5) in his account of the embryology 

 of the hive-bee, confirming the early account given by 

 B. Grassi (1884) on the development of the same familiar, 

 insect and that of J. Carriere and O. Burger (1897) in their 

 full description of the embryology of the Mason Bee 

 (Chalicodoma). These last observers state that the two 

 endodermal rudiments which give rise to the mid-gut in the 

 mason bee embryo appear in those regions of the germ- 

 band v/here the two ectodermal inpushings produce later 

 the fore-gut and the hind-gut (Fig. 41 , A). It may perhaps 

 be possible to find reconciliation between the second and 



