EMBRYOLOGY 201 
egg. In a vertical section along a line which will afterwards 
become the axis of the embryo, the whole headfold is in the shape 
of an ©. The authors named above ingeniously suggest the 
making of a rough model in order to render the somewhat compli- 
cated matter easier to comprehend. Spread a cloth ont flat to 
represent the blastoderm, and by placing the left hand underneath 
it mark the axis of the embryo, and then tuck in the cloth from 
above under the tips of the fingers. The fingers, covered with the 
cloth and slightly projecting from the level of the rest of the cloth, — 
will represent the head, in front of which will be the semicircular 
or horseshoe-shaped groove of the headfold. 
A similar, but shallower fold, appears at the hind end of the 
embryo. This, the “tailfold,” travelling forwards and the “headfold” 
gradually extending backwards, and a pair of lateral folds uniting 
the two and moving inwards, ultimately succeed in forming a 
A.R. Am 
TRANSVERSE SECTIONS THROUGH THE TRUNK OF AN EMBRYO ON THE THIRD AND S1xTH Days. 
A,F. Anterior amniotic fold ; Al. Allantois ; Am. Amniotic cavity; Ch, Chorda dorsalis ; 
m, Spinal marrow ; Se. Membrana serosa. 
tubular sac seated upon and connected by a continually-narrowing 
hollow stalk, with that larger sac which is formed by the extension 
of the rest of the blastoderm over the whole yellow yolk. The 
smaller or upper sac contains, or rather forms the embryo, the 
larger or lower sac is the yolk-sac. As incubation proceeds the 
contents of the yolk-sac are gradually assimilated by nutritive 
processes into the tissues forming the growing walls of the embry- 
onic sac. Consequently the latter becomes larger and larger at 
the expense of the former. Within a few days of the hatching of 
the chick, when the embryo is nearly complete, the yolk-sac is still 
of some considerable size, and is slipped into the body of the 
embryo through the umbilicus or navel. In the article ALTRICES 
it has been pointed out that in the Nidifuge a considerable 
amount of this yolk still exists when the embryo is hatched, while 
in the Nidicole this food-yolk has been completely, or nearly so, 
used up by the time the embryo is ripe. 
The whole mass of the white of the egg, between the shell and 
