390 



CHILOGNATHA. 



the three existing pairs of limbs there are developed three fresh 

 pairs, of ivJiicJi tJic tit'o anterior belong to a single primitive seg- 

 ment. While the above changes take place in the appendages 

 the embryo undergoes an ecdysis, which gives rise to a cuticular 

 membrane within the single egg-membrane (chorion, Metschni- 

 koff). On this cuticle a tooth-like process is developed, the 

 function of which is to assist in the hatching of the embryo 

 (fig. 174 A). 



In Polyxenus a cuticular membrane is present as in Strongylosoma, 

 but it is not provided with a tooth-like process. In the same form amoeboid 

 cells separate themselves from the blastoderm at an early period. These 

 cells have been compared to the embryonic envelopes of Insects described 

 below. 



In Julus two cuticular membranes are present at the time of hatching : 

 the inner one is very strongly developed and encloses the embryo after 

 hatching. After leaving the chorion the embryo Julus remains connected 

 with it by a structureless membrane which is probably the outer of the two 

 cuticular membranes. 



At the time when the embryo of Strongylosoma is hatched 

 (fig. 174 B) nine post-cephalic segments appear to be present. 



FlG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII. 



(After Metschnikoff.) 



A. A seventeen days' embryo, already segmented. 



B. A just-hatched larva. 



Of these segments the second is apparently (from Metschnikoff's 

 figure, 174 B) without a pair of appendages; the third and 



