SIFHONAPTERA AND DIFTERA 



375 



mes 



I— fr.h 



already been stated, this part gives rise only to the mesoderm which by 

 the deepening of the longitudinal ventral furrow later forms a tube 

 (Figs. 328C, 329^). 



Escherich (1900) especially emphasized the point that the mesoderm 

 near the anterior end of the embryo originates below the ectoderm as 

 lateral diverticula of the gastral furrow (Fig. 329C), thus conforming with 

 the mode of development exemplified by the Chaetognatha (Sagitta) 

 (Fig. 40), except that in the muscid it is restricted to the posterior 

 extremity of the very short anterior mid- 

 gut rudiment, the middle strand being 

 absent. 



The tube is first completed at the 

 anterior and posterior ends. Anteriorly 

 the invagination is complicated by the 

 appearance of diverticula (Figs. 3295, C, 

 330). During this time, because of the 

 growth of the embryo and the lengthening 

 of the tubular mesoderm, the posterior 

 mesenteron rudiment together with the 

 germ cells is pushed over on the dorsal side 

 (Figs. 329B,D). The transverse lines now 

 deepen and extend around the egg. With 

 the elongation of the tubular mesoderm 

 these transverse furrows become displaced 

 so that they occupy a more or less oblique 

 position (Fig. S2SD). This is the only 

 indication of rotation that is observable in 

 the embryo of the muscids. The anterior 

 converging folds now elongate still more 

 as well as coming closer together, with the 

 result that the anterior end of the germ 

 band is more sharply defined. 



The original posterior transverse furrow which is now carried by the 

 dorsad-growing posterior section of the germ band becomes more longi- 

 tudinal in position, so that eventually it becomes a longitudinal furrow 

 (Fig. 328D). The transverse lines which originally were found only on 

 the ventral side now become more evident on the dorsal side and finally 

 disappear on the ventral side entirely. When the posterior transverse 

 furrow assumes a longitudinal position, the two branches come closer 

 together and then fuse (Fig. 328Z)). Meanwhile just in front of this, i.e., 

 dorsad, a transverse invagination, the amniotic fold, appears, the first 

 ectodermal invagination (Figs. 331 A, B). 



Fig. 330. — Ccdliphora vomitoria. 

 Frontal section same age as Fig. 

 329. (ect) Ectoderm, {ent) Ento- 

 derm, (/r) Oblique head furrow 

 as in Fig. 328 A. (/r. h) Horizontal 

 head furrow, (mes) Mesoderm. 



