COLUMBA. 381 
pro-amnion. On the second day there arises a fold of the blastoderm 
in front of the embryo, called the head-fold of the amnzon. Similar 
lateral folds and a tail-fold all meet above and fuse together. The 
inner portions of the fold form the amzzon, completely enveloping the 
embryo in a sac, and the outer portions are part of the serous membrane. 
The amnion by its formation is clearly lined with epiblast and covered 
with mesoblast. It contains a fluid gawor amnzz and envelops the 
embryo till hatching. The mesoblast has already split into somatic and 
splanchnic layers before the formation of the amnion. As this split is 
continued downwards round the yolk-sac, it divides the wall of the yolk- 
sac into serous membrane and inner yolk-sac membrane. The amnion 
is completely formed on the fourth day, but the serous and yolk-sac 
membranes are not completely separated till about the seventeenth day. 
The embryo becomes pinched off from its yolk-sac in much the same 
way as in the skate, and the general origin of the organs is much as 
described in the general account of the Vertebrata, 
The last foetal membrane to appear is the allantois. Traces of it 
occur on the second day, but it grows out from the embryo on the 
fourth and fifth days. It is a median ventral diverticulum of the hind- 
gut and hence is lined with hypoblast covered with mesoblast. It 
spreads between the amnion and the dorsal wall of the serous membrane. 
Its walls are covered with branches of an allantoic artery and vein and 
it acts as a breathing organ, its cavity serving as a urinary bladder. It 
has been compared with the urinary bladder of the frog. The yolk-sac 
membrane also has yitelline arteries and veins which serve to absorb 
the yolk. In the later stages, the yolk-sac also absorbs the albumen, 
apparently through the serous membrane. On the twenty-first day the 
yolk-sac is absorbed, the chick breaks its way first into the air-chamber 
and inflates its lungs, and then breaks its shell. It ruptures the amnion 
and the remains of the allantois adhere to the inner surface of the shell. 
We may note that the development of the chick, like that of the 
skate, is purely embryonic, with a lecithal and albuminal nutrition. In 
contrast with the skate and frog, we note the incubation by the mother 
and the presence of amnion and allantois. 
