CHAPTER III. 



THE CHANGES WHICH TAKE PLACE DURING THE FIRST 

 DAT OF INCUBATION. 



During the descent of the egg along the oviduct, 

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

 gertliinal disc, as we have seen, undergoes important 

 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 

 temperature of natural or artificial incubation, the vital 

 activities of the germ are brought back into play, the 

 arrested changes go on again, 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 



