DEVELOPMENT OF GALLUS 211 



cavity, covered over by two membranes of which the inner 

 is the amnion and the outer is the chorion. Both these 

 membranes are of course part of the extra- embryonic ectoderm, 

 and the fact that there are two of them is due to the amniotic 

 fold having two layers as it rises up. At the hind end, the 

 amnion and chorion remain in contact at their point of fusion, 

 forming the so-called sero-amniotic connexion. Extra- 

 embryonic mesoderm gets carried up with the ectoderm in 

 the amniotic fold, and forms a layer on the outer side of the 

 amnion and on the inner side of the chorion. The space 

 between the amnion and the chorion is therefore occupied 

 by extra- embryonic coelomic cavity. The amniotic cavity is 

 of course lined by ectoderm, and contains fluid. Although 

 laid on dry land, therefore, the chick embryo develops in a 

 fluid medium which may be said to be an artificial " pond," 

 equivalent to the pond in which the (more aquatic) ancestors 

 developed, as the frog now does. The embryo is also pro- 

 tected by the amnion as by a water cushion from shocks and 

 knocks to which the shell may be subjected, and from too 

 sudden changes of temperature. 



The Gut. — The base of the amnion grows in beneath the 

 embryo, thus accentuating the folding off of the latter from the 

 rest of the blastoderm. The head-fold has already been 

 noticed, and as the base of the amnion grows in beneath it and 

 backwards, a floor is formed for the most anterior region of the 

 gut. In the same way at the posterior end, a tail-fold develops, 

 and the base of the amnion growing in forms a floor for the 

 hindmost region of the gut. The middle portion of the 

 gut has as yet no floor, and is directly open to the surface of 

 the yolk underneath ; its sides are formed, however, and it is 

 known as the intestinal groove. The passage between the 

 intestinal groove and the foregut, over the edge of the floor 

 of the latter, is called the anterior intestinal portal. Similarly, 

 at the hinder end a posterior intestinal portal is formed. At 

 first the two intestinal portals are far apart, which is the same 

 thing as saying that the formation of the floor of the gut has 

 not yet proceeded very far back from the front end or forwards 

 from the hinder end of the embryo. As development goes 



