230 



MYRIOPODA. 



to the anterior portion, the ventral surfaces of the one half turned 

 towards that of the other (Fig. 116 A and B). 



In the Chilopoda, we were able to trace the inward flexure of the 

 germ-hand to the fact that, during the original dorsal curvature 

 of the long germ-band which extends almost entirely round the 

 egg, the development of the dorsal surface is not possible, and 

 consequently the change to the ventral curvature takes place. In 

 consequence of the length of the embryo, the latter is obliged to 

 assume 'a bent position within the egg. The germ-band of the 

 Diplopoda, however, is short, and the dorsal surface might very 

 well develop without the intervention of the ventral flexure. "We 

 nevertheless find the formative processes met with in the Chilopoda 

 recurring in the Diplopoda, and the conditions which, in the former 



■A "-- ' 



Fig. 116.— Two embryos of Julus Moreletti, illustrating the ventral flexure and the sinking of 

 the germ-band into the yolk (after Metschnikoff). at, antenna ; d , yolk ; kl, cephalic lobe ; 

 md, mandible ; x>i~Px> first three pairs of legs ; si, caudal lobe ; uk, maxilla. 



case, were mechanically necessary for the development of the long 

 embryo, are here perhaps rather adapted to the support of the 

 embryo. It is also possible that the larger extent of surface thus 

 brought into contact with the yolk facilitates the nutrition of the 

 embryo. The ventral flexure was thus retained, although its primitive 

 significance is lost. These processes are of special interest when 

 compared with the sinking of the germ-band into the yolk in 

 Peripatus (footnote, p. 216) and in the Insecta. 



In Julus the invagination of the germ-band takes place only after the rudi- 

 ments of the antennae, the mouth-parts and three pairs of legs have appeared 

 (Fig. 116 A), but in the Strongijlosouia, Polydesmus, and Polyxcnus it occurs at a 



