354 EMBRYOLOGY. 



a very remarkable, hard, horny process near its point, evidently 

 to enable the young creature to break through the shell, for 

 the process in question falls off very shortly after the escape 

 of the bird. The labour of getting free from the shell gene- 

 rally lasts half-a-day ; at length the upper part is raised, the 

 chick pushes out its feet, draws its head from under its wing, 

 and erecting itself quits the shell completely. The remain- 

 der of the chorion and amnion, which, with the closure of 

 the umbilicus, could no longer be nourished, shrivel, fall off, 

 and are left behind in the shell. 



PHYSICAL AND CHEMICAL CHANGES IN THE EGG DTJKING 



INCUBATION. 



[ 499. Various physical and chemical changes take place in 

 the egg during the period of incubation. It loses weight : in 

 the first week, to the extent of five per cent. ; in the second, 

 the amount is thirteen per cent. ; and in the third, sixteen per 

 cent. So that an incubated egg, with an embryo ready to 

 emerge from it, is altogether lighter than one that is just laid; 

 a new-laid egg sinks in water, an egg at the end of the 

 period of incubation swims. The cause of this loss of weight 

 lies in the evaporation of the watery part of the albumen ; 

 the same thing happens, though more slowly, in unincubated 

 eggs from keeping ; the greater rapidity of the loss in the 

 incubated egg arises merely from the greater heat to which it 

 is subjected. Another consequence of the evaporation is the 

 formation and rapid enlargement of the air-space, which, as 

 we have seen ( 477), is first produced after the egg is laid. 

 It is probable that the evaporation in question is connected 

 with chemical changes, for the air contained in the blunt end 

 of the egg is not simple atmospheric air, but contains a 

 larger proportion of oxygen, the amount varying between 

 twenty-five and twenty-seven per cent. This hyper-oxyge- 

 nated air serves the embryo in the process of respiration, or 

 aeration, that is carried on by the medium of the allantois ; 

 for eggs may be incubated to the perfect maturity of the ein- 

 bryo, even without the contact of the external atmospheric 

 air, and may be hatched alike well in pure oxygen and in va- 

 rious irrespirable gases ; for example, pure hydrogen, nitro- 

 gen, &c. At the beginning of the incubation the fluid albu- 

 men contains a small quantity of oil, apparently communicated 



