DIPLOPODA — THE FURTHER DEVELOPMENT OF THE EMBRYO. 



231 



very early stage, the change from the condition in the Chilopoda, which is 

 regarded by us as the primitive condition, having advanced still further than in 

 Julus. 



Of these genera, Strongylosoma has been the best investigated, and Metsch- 

 xikoff has shown that the first indication of the germ-band appears almost in 

 the way that has been described. A transverse furrow, however, seems to appear 

 very early, even before a trace of limb-rudiments is visible. The furrow deepens 

 as in Geophilus and Julus (Fig. 117 A), and here also leads to the ventral 

 curvature of the germ-band (Fig. 117 B). In Strongylosoma the germ-band 

 does not sink in deep, but in Polyxenus it projects somewhat further into the 

 yolk. We can also make out from Fig. 117 5, that the whole of the germ-band 

 does not sink into the yolk, as the most anterior and posterior parts of it 

 (cephalic and caudal ends) still remain on the surface. This seems indicated 

 in Julus also (Fig. 116). 



Besides the transverse furrow, Metschxikoff also observed a longitudinal 

 groove extending 

 far forward (and 

 no doubt back- 

 ward as well) over 

 the germ - band. 

 This groove, 

 which is here 

 somewhat deep, 

 corresponds to 

 that described by 

 Sograff in Geo- 

 philus as appear- 

 ing in early stages, 

 but it seems to be 

 much more dis- 

 tinct in Strongy- 

 losoma than in 

 Geophilus (p. 222). 



The antennae 

 and mouth-parts 

 very soon develop 

 on the anterior, 



sunk portion of the germ-baud, and the first pair of legs develop posteriorly. 

 In this way a stage is reached similar to that attained by the bending of the 

 germ-band of Julus when already provided with limb-rudiments. 



Fio. 117. — Embryos of Strongylosoma Guerinii (A) and Polyxenus 

 lagurus (B), to show the early invagination of the germ-band before 

 the appearance of appendages. The dorsal part of the yolk has 

 been omitted. In B the lamination of the germ-band due to the 

 formation of the germ-layers can be seen (after Metschnikoff). 

 d, yolk; eh, egg-shell; ek, point at which the germ-band (Jest) is 

 flexed. 



The further Development of the Embryo. 



Various factors co-operate to bring about the transformation of 

 the ventrally flexed embryo, sunk in the yolk, into the adult form. 

 By the growth of the germ-band towards the dorsal surface, and its 

 simultaneous extension anteriorly and posteriorly, the yolk is taken 

 up into the embryo. The ventral surfaces of the anterior and 

 posterior portions of the body, till now approximated, move apart, 

 and the whole embryo, the dorsal surface of which has also now 



