626 



BIRDS. 



folds now consist of a double layer of somatopleure, covered externally 

 by epiblast. The folds meet above the back of the embryo and coalesce. 

 The inner layer forms the true amnion, the outer the false amnion or 

 subzonal membrane. Into the space between the amniotic folds, a diver- 

 ticulum from the posterior region of the gut, the allantois, grows out. 



Before the end of the first day, blood vessels begin to be developed 

 in the extra-embryonic region of the blastoderm. These form the 

 beginning of the vitelline vessels, which are of great importance in the 

 early stages of development, and have probably at first some respiratory 

 importance. As development proceeds, the allantois increases greatly, 



and, fusing with the subzonal mem- 

 brane, approaches close to the egg- 

 shell. It has a large blood supply, 

 and functions as an organ of respira- 

 tion ; in addition it absorbs the white 

 of egg> thus serving as an organ of 

 nutrition ; it also receives deposits 

 of urates, thus functioning in con- 

 nection with excretion. 



We have spoken of the "folding 

 off" of the embryo ; it is important 

 to realise that, as a result of this, 

 the still small embryo is attached 

 by a relatively narrow stalk to the 

 large yolk-sac, over which the blasto- 

 derm is now slowly spreading. In 

 this respect the embryo strongly 

 resembles that of the dog-fish ; it 

 differs from the latter in the pre- 

 sence of the over-arching amniotic 

 folds, and in the respiratory allan- 

 tois, which functionally replaces the 

 external gills of the young dog-fish. 

 In the young tadpole the yolk lies 

 heaped up on the floor of the gut, 

 and causes a certain amount of dis- 

 tortion. In the chick, as in the 

 embryo dog-fish, the amount of yolk 

 is so great that it forms a hernia- 

 like protrusion of the gut, and only at a very late stage is the greatly 

 reduced sac withdrawn into the body cavity, after which the dermal and 

 intestinal umbilical openings are closed. 



With regard to the development of the various organs of the body, the 

 conditions are much the same as for the frog. The chick embryo never 

 exhibits any trace of gills, but the gill-clefts perforate the pharynx. The 

 embryonic organ of respiration is the allantois, but that arrangement of 

 aortic arches by means of which in the tadpole blood is carried to the 

 gills, is repeated here. 



About the twentieth day the beak petforates the membranes of the 

 air-chamber, and, the air rushing in, expands the hitherto functionless 

 lungs. At the same time important changes occur in the circulatory 



Fig. 276. — Diagrammatic section 

 of embryo within egg. — After 

 Kennel. 



D., Yolk-sac ; d., wall of yolk-sac ; da., 

 gut of embryo; al., at'., inner and 

 outer wall of the allantois; ajn., 

 amnion proper (the reference line 

 should extend further inwards); a., 

 within amniotic cavity; s., subzonal 

 membrane ; /. is placed within the 

 extra-embryonic body cavity into 

 which the allantois grows. 



