full set is laid, and yet that all of the set hatch on or about 

 the same time. This statement uncovers a possibility which 

 is of interest, and it is also one needing further investiga- 

 tion; it is quite possible that the first eggs laid in such a 

 set undergo a slow but appreciable development, a speed 

 of embryonic growth which is perhaps never quite equal to 

 that of the later fresher eggs of the set, yet the amount of 

 development in the first eggs make up for the lack of 

 developmental speed and all the eggs in such a set mature at 

 the same time. One must not forget that there is an ap- 

 preciable degree of development of the embryo before an 

 egg is laid, and that it is probable that the speed of this 

 "uterine" development is continued at the same, or at an 

 accelerated speed if the egg be incubated at once after being 

 laid, and, contrariwise, if this initial development be sus- 

 pended for a considerable time after the egg is laid, the 

 developmental process is slow in speeding up and in getting 

 under way again, all of which apparently elongates the true 

 incubation length. It is said (49) that perfectly fresh turkey 

 eggs, if incubated at once, hatch a "few hours earlier" than 

 older eggs. This whole question of the behavior of 

 fresh and older eggs, especially of wild birds, under incu- 

 bation conditions needs attentive and thorough investiga- 

 tion, especially through the channels of experimental 

 methods. 



Influence of the Shell 



How much difference, if any, variations in the egg shell 

 make in the duration of incubation of birds' eggs under 

 natural conditions is unknown to me, and it seems, a priori, 

 that this question would be exceedingly difficult to settle. 

 It is a well-recognized factor producing variations in the 

 success of artificial incubation of hens' eggs, since all poultry 

 raisers advise against mixing the eggs of different poultry 

 breeds in the same incubator, alleging deciding want of suc- 

 cess with such practice, and ascribing this to different de- 

 gress of thickness, hardness, coarseness, porosity, etc., of the 

 shell of different breeds; this assumption probably has some 

 truth, for these differing conditions of the shell might affect 

 the readiness with which such eggs respond to the heat of 

 the incubator. It is probable that shePs thinner than normal 

 would permit of too rapid radiation when uncovered, which, 

 if prolonged, would result in the embryonic growth being 

 slowed down, only adding hours or days to the true length of 

 incubation, and not causing a permanent modification of this 

 specific length. The proof that the shell of eggs of wild 

 birds varies is too clear to be overlooked, and its possible 

 modifying effect on the length of incubation cannot be dis- 

 regarded. However, Meyer (178) has shown that the egg 

 shell of the song sparrow can vary 46% in the weights of 



37 



