4io 



ARTHROPODA. 



thickenings of the procephalic lobes (fig. 244), which eventually 

 separate from the superficial skin. There is formed however in 



FIG. 242. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.) 

 The figure shews the jaws (mandibles), and close to them epiblastic involutions, 

 which grow into the supracesophageal ganglia. The antennas, oral cavity, and oral 

 papillae are also shewn. 



each of them a semicircular groove (fig. 244, gr) lined by the 

 superficial epiblast, which becomes detached from the skin, and 

 is involuted to form part of the ganglia. 



A similar mode of formation of both the ventral cords and 

 the supraoesophageal ganglia obtains in Insects (fig. 245). The 



FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA 



LABYRINTHICA. 



The ventral cords have begun to be formed as thickenings of the epiblast, and the 

 limbs are established. 



me.s. mesoblastic somite; vn. ventral nerve-cord; yk. yolk. 



ventral cords are however much less widely separated than in 

 Spiders, and early unite in the median line. In the supraoeso- 

 phageal ganglia the invaginated epiblast has in Lepidoptera 

 (Hatschek) the form of a pit on the dorsal border of the 

 antennae. 



