412 WILSON. [Vot. III. 
2. Stomodwum. — The foundation of the stomodzeum by con- 
crescence of the ectoblastic layer of the germ-bands has already 
been described; its late development is shown in Figs. 79-82. 
As the stomodzeum grows in, its cells become ciliated, consider- 
ably enlarged, and their substance becomes extensively vacuo- 
lated. Somewhat later the walls of the stomodzeum become 
several cells deep, consisting of a deeper stratum of small gran- 
ular cells and an irregular superficial stratum of clear ciliated vac- 
uolated cells, which are often dovetailed in with the smaller cells, 
but sometimes lie flat on their outer ends. Still later the larger 
cells disappear completely, and the stomodzeum consists only of 
columnar, granular cells derived from the smaller deeper cells. 
The stomodzum grows inwards exactly between the meso- 
blastic cavities of the first somite (which li at its sides), and ven- 
tral to the head-cavity. It grows thence backwards as far as the 
dissepiment between the fifth and sixth somites, where its limit 
can be distinctly made out in vermiform embryos 15 mm. long 
and nearly ready to hatch. In such embryos the circum-ceso- 
phageal vessels are fully established, so as to show clearly the 
limit between the pharyngeal and cesophageal regions. The first 
aortic arch, which is smaller than the following ones, is found 
in the sixth somite, immediately behind the limit of the stomo- 
dzum. It is clear therefore that the stomodzeum gives rise 
only to the pharynx, and that the cesophagus, with its calciferous 
glands, is derived from the archenteron, —a result in accordance 
with Vejdovsky’s account of the stomodzum in Rhyuchelmts 
(No. 44, p. 100). In front of the sixth somite the dissepi- 
ments break up more or less completely to form a mass of 
radiating fibrous structures which suspend the pharynx in a 
large irregular cavity which extends forward about to the second 
somite. Still further forward the ccelom is wholly obliterated, 
being filled, like the prostomial cavity, with a spongy mass of 
muscles, connective tissue, vessels, and nerves. 
I may at this point conveniently speak of the change of posi- 
tion which the cephalic ganglia undergo during the differentia- 
tion of the stomodzum. When first formed, these ganglia lie, 
as we have seen, at the extreme anterior part of the body in 
the prostomial cavity. But as the stomodzeum grows inwards the 
ganglia travel gradually backwards until they come to lie in the 
third somite in the anterior extremity of the ccelom. From 
