EMBRYOLOGY 205 
alimentary canal, and is consequently lined inside by hypoblast. 
The sac extends forwards into the peritoneal cavity, until it reaches 
the stalk connecting the embryo with the yolk-sac, whereupon it 
grows rapidly, and pushes its way into the space between the true 
and false amniotic sacs. Curving over the embryo, the allantois 
comes to lie partly above the embryo, separated from the shell by 
nothing more than the thin false amnion. Being thus situated most 
superficially, and in close proximity to the air which penetrates the 
porous shell, the allantois, besides acting as a receptacle for the 
urine, becomes highly vascular, and performs the functions of a 
respiratory organ. Towards the end of incubation, when the 
embryo is already able to breathe through its lungs, the allantois 
shrivels up and is cast off, together with the shell, but its narrowed 
and elongated stalk, from the gut to the navel, remains for some 
time as the urachus upon the inside of the abdominal wall. 
Chronological and Special Account of the Development of the Embryo 
of the Common Fowl. 
First day. 1st to 8th hour of incubation.—Scattered cells appear 
between the epiblast and hypoblast, as the beginning of the middle 
layer or mesoblast ; they are confined to the posterior part of the 
area pellucida, and cause this part, called now the embryonic shield, 
to become somewhat opaque. 
8th to 12th how.—The three embryonic fundamental layers 
are more distinctly established ; the embryonic shield grows fainter, 
and vanishes after there has appeared within it, through a thicken- 
ing of the median portion of the blastoderm, the primitive streak, which 
is a structure of significance still little understood. The hitherto 
pellucid area becomes oval, its narrow end corresponding with the 
future hind end of the embryo. If an egg be placed with its broad 
end to the right hand of the observer, the head of the embryo will 
in nearly all cases be found pointing away from him. 
12th to 16th howr.—The pellucid area becomes pear-shaped ; the 
primitive streak is marked by a shallow median longitudinal furrow, 
known as the primitive groove. 
16th to 20th howr.—An important structure, the notochord, found 
in all vertebrate animals, makes its appearance in the median line 
in front of the primitive streak. The axial part of the epiblast, 
above the notochord, and in front of the streak, forms two longi- 
tudinal folds, which enclose the medullary groove. In front of this 
groove appears the semilunar headfold, and in front of this again 
the amniotic fold begins to make its appearance. 
20th to 24th houwr.—The semilunar headfold enlarges rapidly, 
and rises above the level of the blastoderm; the medullary folds 
come into contact with each other on the dorsal side, and tend to 
