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 supraœsophageal ganglia. The antennæ, oral cavity, and oral papillæ are also shewn.
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.
A similar mode of formation of both the ventral cords and the supraœsophageal ganglia obtains in Insects ([fig. 245]). The ventral cords are however much less widely separated than in Spiders, and early unite in the median line. In the supraœsophageal ganglia the invaginated epiblast has in Lepidoptera (Hatschek) the form of a pit on the dorsal border of the antennæ.
Hatschek states that there takes place an invagination of a median part of the skin between the two ventral cords, for the details of which I must refer the reader to Vol. II. p. 410. He has made more or less similar statements for the earthworm, but his observations in both instances are open to serious doubt.
