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 various 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 
