BIRDS' EGGS 269 



of organs, such as brain, heart, kidneys, there is a re-treading 

 of the old historical path. Thus the kidneys, which are 

 eventually organs of some compactness, begin as a double 

 row of kidney-tubes, taking our thoughts back to worms. 

 Or, again, every embryo bird has a transitory notochord — 

 the permanent skeletal axis in pioneer Vertebrates like 

 lancelets and lampreys. And every embryo bird has its 

 gill-slits. There are abundant illustrations of the lien that 

 the past has on the present. The fact is that the past lives 

 on in the present, and ontogeny (or individual development) 

 tends in a general way to recapitulate phylogeny (or the 

 evolution of the race). Yet this idea of recapitulation must 

 be held in the mind along with the idea of specificity ; for 

 after a few days have passed the embryo bird is on an avian 

 path, and the duck embryo is different from the fowl- 

 embyro. 



§ 8. Hatching 



When the young chick within the egg is about twenty- 

 one days old, it thrusts its beak into the air-chamber at the 

 broad end of the egg. Air enters the nostrils and fills the 

 lungs for the first time, and in the exhilaration of this first 

 breath the unhatched chick knocks the " egg- tooth " at the 

 tip of its beak against the inside of the shell and breaks a 

 way out. 



Professor Franz Keibel has inquired into the musculature 

 involved in breaking the shell. He finds that the work is 

 done by a special muscle — the musciilus complexus — which is 

 temporarily exaggerated in strength and is used to jerk the 

 beak upwards and backwards against the shell. And just as 

 the egg-tooth — an instrument which only functions once — 

 falls off soon after hatching, so the musculus complexus, 

 having done its work, becomes in a few days relatively 

 smaller. 



Keibel 's account of the hatching of the chick is not borne 

 out by A. G. Pohlman's re-investigation (1919). The 

 musculus complexus does attain a maximum size before and 



