THE ALIMENTARY CANAL AND INTESTINAL GLANDS. 337 



The paired entoderm-bands belong to the lateral parts of the 

 germ-band. They lie, as a rule, immediately below the series of 

 consecutive coelomic sacs (Fig. 158 F, p. 321), where the entoderm- 

 bands (en) are seen cut through below (really dorsal to) the coelomic 

 sacs (us). The dorsal wall of the primitive segments is thus in 

 immediate contact with the entoderm-bands. Active proliferation of 

 cells now takes place from this wall of the primitive segments, and 

 the cell-material thus produced, which splits off from the dorsal wall 

 of the primitive segments, forms the outer layer of the enteron- 

 rudiment, the splanchnic layer or intestinal fibrous layer (Fig. 158 

 F, spm; Fig. 170, sp, p. 344). The remainder of the dorsal wall of 

 the coelomic sac left after this separation enters into relation with 

 the genital rudiment and yields the terminal filament (cf. p. 345, etc., 

 and Fig. 170, ef). The entoderm-bands with the immediately 

 contiguous splanchnic layer may now be termed the mid-gut 

 rudiment (Fig. 150, m, p. 306, and Figs. 170, 171, 172, sp + en, 

 pp. 344-346). In the following stages the enteron is distinguished 

 for its great lateral growth, which causes it to spread over the surface 

 of the food-yolk, which it finally completely surrounds (Fig. 150 

 C-F, p. 306, and Figs. 170-172, pp. 344-346). This circumcrescence, 

 as a rule, occurs in such a way that the two entoderm-bands unite first 

 in the region of the ventral middle line (Figs. 150 F, and 171). 

 Only later do they unite on the dorsal side (Figs. 150 F, and 152). 

 The food-yolk in this way comes to lie entirely within the enteron, 

 and with it is included the remains of the dorsal tube or dorsal 

 organ (Fig. 150 F, s) where such is present. 



The description just given of the development of the enteron, which rests 

 principally on the observations made on Hydrophilus and Pltyllodromia, seems 

 to be directly applicable to most Insects. In individual cases, indeed, we find 

 certain deviations, as, for instance, in Musca, where the coelomic sacs do not 

 attain distinct development (p. 320), and where the whole of the food-yolk is 

 not taken into the enteron, a portion of it remaining (as in other Diptera) in 

 the body-cavity where it is gradually absorbed (Kowalevsky, Voeltzkow, 

 Graber). The conditions also differ to a certain extent in the Hymenoptera 

 (Apis, C'haHcodoina, Kowalevsky, Grassi, Carriere), where the entoderm 

 originally occupies a dorsal position (p. 316) and is only gradually grown over 

 by the germ-band. The circumcrescence of the food-yolk by the entoderm here 

 proceeds from the dorsal to the venti-al side. 



The salivary glands which open into the buccal cavity and may 

 consist of several pairs* (1-3), arise as ectodermal invaginations 



* According to Schiemenz (No. 125), the various cephalic glands of Apis 

 (imago) are distributed in such a way that, originally, a pair occurs on each of 

 the three maxillary segments. 



