CHILOPODA — THE EMBRYONIC ENVELOPE. 



■2-27 



distinct from the yolk-mass upon which it lay, as is evident from a 

 glance at Figs. 109-113. The germ-band now, however, extends 

 laterally and grows round the sides of the yolk-mass, so that the 

 dorsal surface of the embryo begins to develop, and its segmentation 

 commences (Fig. 115). At the same time the two halves of the 

 body that lie parallel to one another lengthen, and approach more 

 and more the final shape, although still showing the ventral curvature 

 (Fig. 115). 



A cuticle was secreted at the surface of the embryo at an earlier 

 stage. When the embryo assumed the ventral curve, the cuticle 

 did not follow that curvature, but bridged it over, and thus 

 remained somewhat separate from the body. In later stages, the 

 body, as well as the anterior limbs, is found sheathed in this cuticle. 

 The mature embryo is still enveloped in it, and it is only cast off 

 after the egg-shell 

 (Fig. 115, eh) has 

 split as the first lar- 

 val integument. In 

 the Geoplrilus inves- 

 tigated by Metsch- 

 nikoff, a tooth is 

 found on the cuticle 

 covering the second 

 maxilla (Figs. Ill 

 and 115, ez), this, ac- 

 cording to Metsch- 

 nikoff, is used for 

 splitting the egg- 

 envelope, and is cast 

 off with the cuticle. 

 \Ye thus have here 

 a recurrence of the 

 structure known as 

 the egg-tooth in the 



Araneae (p. 58). The provisional cuticle in any case corresponds 

 to the envelope formed in other Myriopoda at a still earlier stage 

 which surrounds the embryo in the same way as do the blastodermic 

 cuticle, or the deutovum-membrane of the Acarina (ef. pp. 97 and 234). 



The embryo splits the egg-shell (Fig. 115, eh) at an early stage of 

 development. It still retains the ventral curvature and is sur- 

 rounded by the provisional cuticle. It continues to grow in length 



Fig. 114. — Embryo of Geophilus after completion of the ventral 

 flexure. The ventral surface of the anterior part of the body 

 is turned towards that of the posterior part, and lies almost 

 parallel to it (after Metschnikoff). at, antenna; d, yolk; 

 ck, point at which the germ-band bends ; cz, egg-tooth (on the 

 second maxilla); Id, cephalic lobe ; p, legs; si, caudal lobe. 



