No.3.] ZHE EMBRYOLOGY OF THE EARTHWORM. 4II 
of the stomodzeum and the growth of the anterior somites (Fig. 
50). 
The ingrowing stomodzum (see following section) pushes 
the archenteron backwards as far as the sixth somite, which is 
its extreme limit in the adult and marks the point of union of 
the pharynx and the cesophagus. Thus the cesophagus is lined 
by entoblast, as are also the calciferous glands which develop 
as diverticula from it. At the time of hatching the various 
regions of the alimentary canal are completely differentiated, 
and the typhlosole is established as a deep infolding of the 
dorsal median wall of the intestine. I have not yet attempted 
to follow in detail the histological differentiation of the ali- 
mentary wall. 
VIII. THe EcrospraAst’anp ITs) PRODUCTS. 
1. The origin of the general ectoblast of the ectoblastic stra- 
tum of the germ-bands and of the stomodzeum have already been 
described. At the end of the gastrulation the ventral aspect of 
the embryo is covered by a large-celled pavement epithelium 
which passes abruptly at the sides into the swollen superficial 
granular cells of the germ-bands. A little later some of the 
cells along the median ventral line become enlarged and covered 
with short cilia which cause the embryo to rotate slowly in the 
longitudinal vertical plane. The ciliated cells are at first arranged 
in a single irregular series along the median line, and the indi- 
vidual cells are separated from one another by groups of smaller 
non-ciliated cells. Ata later period the ciliated cells come into 
contact and form an irregular continuous double row which form 
the floor of a slight but distinct ventral groove. In front this 
groove nearly disappears, but the ciliated cells are continuous 
with the ventral lip of the stomodzeum, and the cilia extend 
around the lips of the stomodzeum into its cavity. In still later 
stages the ciliated cells gradually disappear, apparently by break- 
ing up into ordinary ectoblastic cells and losing their cilia. I 
am certain that they take no part in the formation of the ventral 
nerve-chain, which is entirely formed by concrescence of the two 
neural rows. As the germ-bands spread around the body, the 
pavement-epithelium of the dorsal and ventral surfaces is grad- 
ually converted into a columnar epithelium which persists as the 
hypodermis and is covered by the cuticle. 
