where the
allantois is shaded. Allantoic bloodvessels ramify thickly over its
walls, and aeration occurs through the permeable shell.

Section 29. The nature of the amnion will be understood by following
Figures 4b, 5, and 6 on Sheet 23. The three embryonic layers are
indicated by broken lines, dots, and black lines, just as they are in
the frog diagrams. Not only is the embryo slowly pinched off from the
yolk sac (y.s.), but, as the yolk is absorbed beneath it, and it grows
in size, it sinks into the space thus made, the extra-embryonic
somatopleur and epiblast rise up round it as two folds, which are
seen closing in 5, and closed in 6, over the dorsal side of the young
chick. In this way a cavity, a., lined by epiblast, and called the
amniotic cavity, is formed. Dorsal to this, in 6, comes a space lined
by somatic mesoblast, and continuous with p.p., the
pleuro-peritoneal cavity, or body cavity of the embryo. Outside this,
again, is a layer, of somatopleur internally and epiblast externally, the
false amnion (f.a.), which is continuous with the serous membrane
(s.m.) enclosing the rest of the egg. The student should, carefully
copy these diagrams, with coloured pencils or inks for the different
layers, and should compare them with the more realistic renderings of
Figures 2, 5, and 8, Sheet 24.

Section 30. The heart in the fowl appears first as a pair of vessels,
which unite to form a straight trunk in the median line, as the
flattened-out embryo closes in from the yolk. The way in which this
straight trunk is thrown, first of all, into the S shape of the fish heart,
and then gradually assumes the adult form, is indicated roughly by
Figure 3. In one respect the development of the heart does not follow
the lines one would expect. Since, between the fish and the higher
form comes the condition of such an animal as the frog, in which the
auricles are divided, while there is only one ventricle, we might expect
a stage in which the developing chick's heart would have one ventricle,
and a septum between the auricles. But, as a matter of fact, the
ventricles in fowl and rabbit are separated first, and the separation of
the auricles follows, and is barely complete at birth.

Section 31. Two vitelline veins from the yolk sac (v.v.) flow into the
heart from behind, as shown in Figure 1. A later more complete and
more diagrammatic figure of the circulation is seen in Figure 7. At first
there are two anterior cardinal (a