. Outlines of zoology. .1., subzonalspect the embryo strongly resembles membrane; I. is placed within thethat of the dogfish; it differs from extra• embryonic body cavity intothe latter in the presence of the ^^ich the allantois grows,overarching amniotic folds, and in, the respiratory allantois, which functionally replaces the protrudinggills of the young dogfish. In the young tadpole the yolk lies heapedup on the floor of the gut, and causes a certain amount of the chick, as in the embryo dogfish, the amount of yolk is so greatthat it forms a hernia-like protrusion of the gut,
. Outlines of zoology. .1., subzonalspect the embryo strongly resembles membrane; I. is placed within thethat of the dogfish; it differs from extra• embryonic body cavity intothe latter in the presence of the ^^ich the allantois grows,overarching amniotic folds, and in, the respiratory allantois, which functionally replaces the protrudinggills of the young dogfish. In the young tadpole the yolk lies heapedup on the floor of the gut, and causes a certain amount of the chick, as in the embryo dogfish, the amount of yolk is so greatthat it forms a hernia-like protrusion of the gut, and only at a very latestage is the greatly reduced sac withdrawn into the body cavity, afterwhich the dermal and intestinal umbilical openings are closed. The chick embryo never exhibits any trace of gills, but the gill-cleftsperforate the pharynx. The embryonic organ of respiration is theallantois, but that arrangement of aortic arches by means of which inthe tadpole blood is parried to the gills is repeated 686 BIRDS. About the twentieth day the beak, which has a hard tooth on thetip, perforates the membranes of the air-chamber, and the air, rushingin, expands the hitherto functionless lungs. At the same time import-ant changes occur in the circulatory system, the umbilicus becomescompletely closed, the allantois shrivels up, and the chick, piercingthe broad end of the shell with repeated blows of its beak, steps outinto the world. Classification of Birds I. Sub-Class or Saurur^. Ancient extinct birds,connecting Birds and Reptiles The oldest known bird is Archaopteryx, two specimens of whichhave been found in the Solenhofen Lithographic Stone (Upper Jurassic)of Bavaria. The stone is so fine-grained that, besides the bones ofthe wings, the furculum or merrythought, the pelvis, the legs, and thetail, we have actually casts or impressions on the stone (made when itwas as yet only soft mud) of all the feathers of the wings, and of thetail.—Nicholson and
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
