3i>2 CHILOGNATHA. 



ments, unless the reverse is stated, apply to the species of Stron- 

 gylosoma 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 appendages, which 

 become the antenna?, makes its appearance shortly after the forma- 

 tion 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 thickening has in the meantime become marked by a longi- 

 tudinal 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 appen- 

 dages behind the antennas two further pairs become added, and at the 

 same time oral and anal imaginations become formed (fig. 173 C). 

 In front of the oral opening an unpaired upper lip is developed. 

 The prse-oral part of the ventral plate develops into the bilobed pro- 

 cephalic lobes, the epiblast of which is mainly employed in the 

 formation of the supra-cesophageal 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 

 backwards, 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 antennas 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 the three existing pairs of 

 limbs there are developed three fresh pairs, of which the two anterior 

 belong to a single primitive segment. 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-niembraue 

 (chorion, Metschnikoff). 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 Polyxeuus 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 hatch- 

 ing : the inner one is very strongly developed and encloses the embryo 

 after hatching. After leaving the chorion the embryo Julus remains con- 



