The Bird in the Egg 473 



latter creature there have been found as many as a hun- 

 dred and eighty pairs of gill-clefts, such a remarkable 

 number aerating the blood with but little necessary pro- 

 pulsion, but when in the higher fishes the number of gills 

 in many species is reduced to four, we realize at once the 

 need for a stronger engine to force the blood through the 

 lessened number, this accounting for the increased com- 

 plexity of the heart. 



Up to about the twelfth day the tiny foreshadowings of 

 bones are cartilaginous, like those of the shark, but at this 

 time real osseous, or bony, tissue begins to be deposited 

 in spots which spread rapidly. In the \'arious portions of 

 the skull these bony centres spread until the bones are 

 separated only by narrow sutures, and in the adult bird 

 even these are obliterated, unlike the condition in the 

 skull of a cat or a dog. 



The bones of the adult bird are so neatly joined together, 

 and are so mutually dependent, that we might easily 

 imagine that they were formed in the order of size or 

 importance, or in a regular series, following their connection 

 with one another; but this is not true. The ribs, for 

 example, are formed between the segments of the primitive 

 sheets of muscle, independently of the back-bone, and 

 only later become attached to it. There is no trace of the 

 great keel-bone, or even of the sternum of the adult fowl, 

 until after the ends of the ribs have met in the middle line 

 of the body, when they grow together and give rise to the 

 sternum — a structure not found in fishes. We have 

 learned that the repetition of similar structures (as the 

 ribs) is a sign of a low degree of organization, and the truth 



