6 THE STRUCTURE OF THE FOWL 



quantity of eggs of considerable size. This 

 means that the maintenance of even one ovary 

 is an expensive affair. The retention of two 

 ovaries would doubtless cost too much ; one 

 of them, therefore, dwindles and disappears. 

 Moreover — and possibly this is a better reason — 

 the bird's egg is a large one, and if two ovaries 

 were present and extruded ova at or about 

 the same time, the abdomen would have to be 

 unduly large to contain two fully formed eggs. 



The egg is large because it contains within 

 itself enough food - material to support the 

 growing chick until incubation is ended. The 

 egg as it leaves the ovary is of goodly size and 

 contains a considerable quantity of yolk. But 

 more than 50 per cent of the total weight of 

 the fully formed egg is produced by the physio- 

 logical activity of the oviduct down which it 

 travels on its way from the ovary to the ex- 

 terior. Owing to its large size the egg could 

 not be laid if the two hip-bones were joined 

 together ventrally. In birds, therefore, there 

 is no pelvic symphysis. 



Many other features of avian anatomy — 

 such as the large size of the eye, the single 

 occipital condyle, the right aortic arch — will 

 be revealed during the examination of the 



