TRACHEATA. 389 



The most important sources of information for the general 

 embryology of the Chilognatha are the papers of Newport (No. 

 397) and Metschnikoff (No. 398). The development of Strongy- 

 losoma may be taken as fairly typical for the group ; and the 

 subsequent statements, unless the reverse is stated, apply to the 

 species of Strongylosoma investigated by Metschnikoff. 



After the segmentation and formation of the layers the first 

 observable structure is a transverse furrow in the thickening of 

 the epiblast on the ventral surface of the embryo. This furrow 

 rapidly deepens, and gives rise to a ventral flexure of the embryo 

 (fig. 173 A, x\ which is much later in making its appearance in 

 Julus than in Strongylosoma and Polyxenus. A pair of ap- 

 pendages, which become the antennae, makes its appearance 

 shortly after the formation of the transverse furrow, and there 

 soon follow in order the next three pairs of appendages. All 

 these parts are formed in the infolded portion of the ventral 

 thickening of the blastoderm (fig. 173 B). The ventral thicken- 

 ing has in the meantime become marked by a longitudinal 

 furrow, but whether this is connected with the formation of 

 the nervous system, or is equivalent to the mesoblastic furrow in 

 Insects, and connected with the formation of the mesoblast, has 

 not been made out. Shortly after the appearance of the three 

 pairs of appendages behind the antennae two further pairs become 

 added, and at the same time oral and anal invaginations become 

 formed '(fig- 173 Q. In front of the oral opening an unpaired 

 upper lip is developed. The prse-oral part of the ventral plate 

 develops into the bilobed procephalic lobes, the epiblast of 

 which is mainly employed in the formation of the supra-cesopha- 

 geal ganglia. The next important change which takes place is 

 the segmentation of the body of the embryo (fig. 174 A), the 

 most essential feature in which is the division of the mesoblast 

 into somites. Segments are formed in order from before back- 

 wards, and soon extend to the region behind the appendages. 

 On the appearance of segmentation the appendages commence 

 to assume their permanent form. The two anterior pairs of 

 post-oral appendages become jaws ; and the part of the embryo 

 which carries them and the antennae is marked off from the 

 trunk as the head. The three following pairs of appendages 

 grow in length and assume a form suited for locomotion. Behind 



