COLUMBA. 381 



p7'o-amnion. On the second day there arises a fold of the blastoderm 

 in front of the embryo, called the head-fold of the anuiion. Similar 

 lateral folds and a tail-fold all meet above and fuse together. The 

 inner portions of the fold form the amnion, completely enveloping the 

 embryo in a sac, and the outer portions are part of the serous meinbi-ane. 

 The amnion by its formation is clearly lined with epiblast and covered 

 with mesoblast. It contains a fluid liquor amnii 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 vitelline 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. 



