768 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



in the respiratory allantf)is, which functionally replaces the protruding 

 gills of the young dogfish. 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 dogfish, 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 the greatly reduced sac withdrawn 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 closeci. 



The chick embryo shows gill 

 clefts which perforate the pharynx, 

 and minute vestiges of the an- 

 cestral gills have been described. 

 The embryonic organ of respira- 

 tion is the allantois. 



About the twentieth day the 

 beak, which has a hard " tooth " 

 on the tip, perforates the mem- 

 branes of the air-chamber, and 

 the air, rushing in, expands the 

 hitherto functionless lungs. At 

 the same time important changes 

 occur in the circulatory system, 

 " the umbilicus becomes com- 

 pletely closed, the allantois 



Fig. 460. — Diagrammatic section 

 of embryo within egg. — After 

 Kennel. 



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

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

 outer wall of the allantois ; am., 

 amnion proper (the reference line 

 should extend farther inwards) ; a., 

 within amniotic cavity ; s., subzonal 

 membrane ; /. is placed within the 

 extra-embryonic body cavity into 

 which the allantois grows. 



shrivels up, and the chick, pierc- 

 ing the broad end of the shell with repeated blows of its beak, steps 

 out into the world." 



I. Sub-Class 



Classification of Birds 



Arch^ornithes or .Saurur.«. Ancient extinct birds, 

 connecting Birds and Reptiles 



The oldest known bird is ArchcBopteryx, two specimens of which 

 have been found in the Solenhofen Lithographic Stone (Upper Jurassic) 

 of Bavaria. " The stone is so fine-grained that, besides the bones of 

 the wings, the furculum or merrythought, the pelvis, the legs, and the 

 tail, we have actually casts or impressions on the stone (made when it 

 was as yet only soft mud) of all the feathers of the wings, and of the 

 tail." — Nicholson and Lydekker. 



This link between Birds and Reptiles seems to have been a land bird 

 about the size of a crow. The skull is like that of a typical bird. The 

 upper jaw shows thirteen pairs of conical teeth, the lower about three 

 pairs. They are embedded in sockets. Each of the twenty vertebrae 

 of the long tail bears a pair of lateral rectrices — a unique arrangement. 



