314 



INSECTS, 



from the great lengthening of the gastrula-invagination. We may 

 assume with Rabl* for the median invaginating plate a median 

 unpaired entoderm-band and paired mesoderm-hands. The entoderm- 

 band is, however, dragged apart to form an anterior and a posterior 

 portion by the great lengthening of the furrow (Fig. 154, en, en"), 

 so that over the greater part of the germ-band the two lateral 

 mesoderm-bands meet one another in the median line. 





OTt 



The view just mentioned would receive important support from the statement 

 of Butschli (No. 12) that, in the formation of the germ-layers at the posterior 

 end of the germ-band of Musca, the archenteron actually becomes divided up at 

 a certain stage through the formation of folds into three connected diverticula : 

 of these diverticula, the unpaired median one is to be regarded, just as in 

 Sagitta, as the entoderm-rudiment and the paired lateral ones as the mesoderm- 

 rudiment (coelomic sacs). Since, however, the more recent works on the 

 ontogeny of Musca do not confirm this statement, and the conditions described 

 may, as we shall see, perhaps be interpreted in another sense, we must leave 

 this point for the present undecided. 



The statements made by Kowalevskt (Xo. 49) with regard to the formation 

 of the germ-layers in Musca have been only partially confirmed by the later 

 researches of Voeltzkow (Xo. 85) and Grabek (Xo. 28) on the same animal. 

 According to Voeltzkow, the stomodaeal and proctodaeal invaginations grow 



inwards from the base of the 

 ec „„ gastrula-furrow, and therefore 



belong, not to the ectoderm, 

 but to the lower layer. The 

 anterior and posterior entoderm- 

 rudiments are said to arise by 

 the proliferation of cells from 

 the blind ends of these two 

 invaginations. Graber No. 

 28), indeed, has confirmed 

 Kowalevsky'.s statements for 

 the anterior entoderm - rudi- 

 ment, and also assumes the 

 ectodermal origin for the st"iii<>- 

 daeum. As to the proctodaeum, 

 on the contrary, and the posterior entoderm-rudiment, Graber entirely agrees 

 with Voeltzkow, with the single exception that, for the growth of the pos- 

 terior entoderm-rudiment, he claims not only the blind end, but a long band 

 of the ventral side of the proctodaeum. V\"e may here object to this view of 

 Voeltzkow and Graber that if, in reality, in the Muscidac, a posterior section 

 of the intestine arose by invagination from the lower layer, Ave should not be 

 able to call it the proctodaeum, for in that case we should not be able to regard 

 it as homologous with the similarly-named section of the intestine of other 

 Insects, in which it forms, as in all other animals, from the ectoderm. It, 

 however, appears to us that the sections of the posterior end of the germ-hand 

 of the Muscidae, which are in any case difficult to understand, can be more 



* Theorie des Mesoderms. Morph. Jahrb. 1889. 



Fig. 155.— Diagram illustrating the separation of the 

 germ -layers in the most anterior region of the 

 germ-band of H ydrophilus, transverse section (after 

 Heider). ih, yolk-cells ; ec, ectoderm ; en, ento- 

 derm ; vis, mesoderm. 



