526 THE WONDER OF LIFE 



time ; in tte exhilaration of this fiist breath the unhatched 

 bird knocks vigorously at the shell and breaks open the 

 prison doors. After a few days, in most cases, the egg- 

 tooth, having done its work, falls off — a weU-adapted 

 instrument that functions only once. 



But there is a further detail which is of much interest. 

 The bill and its egg-tooth are only the instruments, what 

 about the musculature which works these ? Professor Franz 

 Keibel has inquired into this in the case of the unhatched 

 chick and duckhng. He finds that the work is done by a 

 muscle called the musculus complexus, and that it is very 

 markedly hypertrophied for some time before hatching. 

 On the tenth day after hatching, it shows no pecuharity. 

 Here then we have a signal instance of the way in which 

 development proceeds as if it were working with a purpose. 

 How comes that musculus complexus to be temporarily 

 exaggerated in strength, in relation to the breaking of the 

 egg-shell — an action which only occurs once in each genera- 

 tion ? 



Similar egg-openers are well known among insects. Thus, 

 in the embryo of a Bug, Palomena dissimilis, described by 

 Heymons, there is on the top of the head a T-shaped 

 chitinous ridge with a minute apical tooth. This curious 

 apparatus is used to force open the hd of the egg. When the 

 young insect creeps out of the egg-envelope, it moults and 

 loses its egg-opener. Thus we have another example 

 of a structure which functions only once in a life-time. 



Before Birth. — There is something very striking in 

 adaptations before birth — in fitnesses which occur while the 

 creature is still at its vita minima and very inert. Mr. 

 T. Southwell finds a good example in the young of the saw- 

 fish {Pristis cuspidoitus). He dissected a large fepiale 



