CHAPTER HI. 



THE CHANGES WHICH TAKE PLACE DURING THE FIRST 

 DAY OF INCUBATION. 



1. During the descent of the egg along the oviduct, 

 where it is exposed to a temperature of about 40" C, the 

 blastoderm, as we have seen, continues to undergo im- 

 portant changes. When the egg is laid and becomes cold 

 these changes all but entirely cease, and the blastoderm 

 remains inactive until, under the influence of the higher tem- 

 perature of natural or artificial incubation, the vital activities 

 of the germ are brought back into play, the arrested changes 

 gro on aoain, and usher in the series of events which we have 

 now to describe in detail. 



The condition of the blastoderm at the time when the 

 egg is laid is not exactly the same in all eggs, in some the 

 changes being farther advanced than in others, though the 

 differences of course are slight ; in some eggs, especially in 

 warm weather, changes of the same kind as those caused by 

 actual incubation may take place, to a certain extent, in the 

 interval between laying and incubation ; lastly, in all eggs, 

 both under natural and especially under artificial incubation, 

 the dates of the several changes are, within the limits of 

 some hours, very uncertain, particularly in the first few days ; 

 one egg being found, for example, at oQ hours in the same 

 stage as another at 24- or 30 hours, or a third at 40 or 48 

 hours. When we speak therefore of any event as taking 

 place at any given hour or part of any given day, we are to 

 be understood as meaning that such an event will generally 

 be found to have taken place at about that time. We 

 introduce exact dates for the convenience of description. 



