DEVELOPMENT OF THE CHICK. 685 
During the course of the second day the embryo seems to sink 
farther into the yolk, while both anteriorly and posteriorly double folds, 
known respectively as the head and tail folds, rise up. In the course 
of their development the embryo becomes completely ‘‘folded off” 
from the yolk. At a slightly later stage, side folds also appear ; all the 
folds now consist of a double layer of somatic mesoderm covered by 
epiblast. The folds meet above the back of the embryo and coalesce. 
The inner layer forms the true amnion, the outer the false amnion or 
subzonal membrane. Into the space between the amniotic folds, a 
diverticulum from the posterior region of the gui, the allantois, grows 
out, 
Before the end of the first day, 
blood vessels begin to be developed 
in the extra-embryonic region of 
the blastoderm. These form the 
beginning of the vitelline vessels, 
which are of great importance in the 
early stages of development, and 
have probably at first some respira-. 
tory importance. As development 
proceeds, the allantois increases 
greatly, and, fusing with the sub- 
zonal membrane, approaches close 
to the egg-shéell. It hasa large blood 
supply, and functions as an organ of 
respiration ;- in addition it absorbs 
the white of egg, thus serving as an 
organ of nutrition; it also receives 
deposits . apes thus oe Fic. 380.—Diagrammatic section 
in connection with excretion. thi at 
We have spoken of the ‘‘ folding oo ni eer eee 
off” of the embryo; asa result of Di Mallsneend walltevalksactaie 
this, the embryo is attached by a ““Zut‘of embryo: aZ, al, inner and 
relatively narrow stalk to the large outer wall of the allantois; aw, 
yolk-sac, over which the blastoderm ee proper ube ee line 
is now slowly spreading. Inthisre- S00." bats ie te Mee eikonal 
spect the embryo strongly resembles membrane; /. is placed within the 
that of the dogfish ; it differs from extra-embryonic body cavity into 
the latter in the presence of the which the allantois grows. 
overarching amniotic folds, and 
in, the respiratory allantois, which functionally replaces the protruding 
gills of the young dogfish. In the young tadpole the yolk lies heaped 
up on the floor of the gut, and causes a certain amount of distortion. 
In the chick, as in the embryo dogfish, the amount of yolk is so great 
that it forms a hernia-like protrusion of the gut, and only at a very late 
stage is the greatly reduced sac withdrawn into the body cavity, after 
which the dermal and intestinal umbilical openings are closed. 
The chick embryo never exhibits any trace of gills, but the gill-clefts 
perforate the pharynx. The embryonic organ of respiration is the 
allantois, but that arrangement of aortic arches by means of which in 
the tadpole blood is carried to the gills is repeated here. 
