202 EMBRYOLOGY 
the vitelline membrane, has soon after the beginning of incubation 
become very fluid and its albumen is like the contents of the yolk- 
sac assimilated into the tissues of the growing embryo. Already 
a few days before hatching it is used up completely, so that by 
this time the embryonic sac and its enclosing membranes fill up 
the whole egg. 
The embryo, as explained above, is formed by a folding-off of 
the portion of the blastoderm from the yolk-sac. The tubular sac 
of the embryo, while everywhere acquiring thicker walls, undergoes 
many modifications through local thickening, budding, and folding, 
and is gradually moulded into the proper shape of the body of 
the chick. 
First there appears, on the upper side, a longitudinal canal, the 
neural tube, the walls of which become transformed into the brain 
and spinal cord. Below and parallel with this tube appears an 
axis represented by the vertebre. Underneath this, again, is 
another tube, closed in above by the axis, and on the sides ‘and 
below by the body-walls. Enclosed in this second tube, and 
suspended from the axis, is a third tube, consisting of the 
alimentary canal with its diverticular appendages, the _ liver, 
pancreas, lungs, etc. The cavity of the outer tube is the body- or 
pleuro- peritoneal cavity; it also contains the heart and other 
parts of the vascular system, together with the genital glands and 
M the kidneys, which are all folded or budded- 
off portions of the inner walls of the body- 
cavity. 
Thus a transverse section of a chick, or in 
=e ® fact of « vertebrate animal, always shews 
uct of any vertebra ; y 
the same fundamental structure; above a 
single tube, below a double tube, the latter 
consisting of one tube enclosed within another, 
the inner being the alimentary canal, the 
DIAGRAMMATIC, TRANSVERSE 
SECTION OF THE Bopy OF ANY 
VERTEBRATE. 
Ao, Aorta; c, Peritoneal 
cavity ; g, Gut-cavity; Gg. 
Genital glands; K, Kidneys; 
M, Spinal marrow contained 
in the vertebral column, the 
vertebre and ribs’ being 
shaded. 
outer the general cavity of the body. Into 
such a triple tube the simple tubular embry- 
onic sac of the chick is converted by a series 
of changes of a remarkable character. 
The upper or neural tube begins at a 
very early period by the raising up of the 
epiblast of the blastoderm into two ridges, 
which run parallel to the long axis of the 
future embryo and enclose a shallow longi- 
tudinal groove. These medullary folds 
eventually meet and coalesce dorsally in the middle line, thus 
converting the groove into a canal which becomes closed at either 
end. The cavity of the tube becomes the cerebro-spinal canal, its 
walls are transformed into the spinal cord and through thickenings 
