. Outlines of zoology. Zoology. 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
. Outlines of zoology. Zoology. 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so great that it forms a hernia- like protrusion of the gut, and only at a very late stage is the greatly reduced sac withdrawn into the body cavity, after which the dermal and intestinal umbilical openings are closed.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 deve
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
