RANKIN'S 'DUCK BOOK 



The discovery and locations of the minute organisms may 

 be interesting to the scientist, but not at all necessary to the 

 operator, who simply wants to be assured. of the life and health 

 •of the germ. This he can readily determine from the increased 

 size and gradual development of the circle ; it, and the con- 

 tents of the egg, now assume a darker shade. Up to this time 

 iuse no moisture, and the contents of the eggs have gradually 

 •evaporated and the air-cell proportionately enlarged. This 

 air-cell is slightly enlarged till the tenth day, when no further 

 •evaporation should take place. About three days before 

 hatching the rapidly developing duck will gradually diminish 

 the size of the air-cell, leaving himself just room enough to 

 •work out. 



Nature, in the case of the old hen, provides for her own 

 •contingencies, while we must resort to art to obtain the same 

 conditions. 



Figure 4. — Egg at End of 96 Hours. 



While incubating under the hen during the first few days, 

 the egg evaporates rapidly. Then the pores gradually become 

 coated with an oily secretion from the feathers of the hen until 

 evaporation ceases. Now, we cannot successfully fill the pores 

 of the eggs, it is too delicate an operation to attempt; but we 

 can easily v obtain the same conditions in another way, and 

 that is to prevent the further evaporation of the egg by vapor- 

 izing water in the egg-chamber, so that evaporation will not 

 take place. Exactly when this should be done is already 

 •known, but exactly how much is quite another thing, and de- 

 pends largely upon the. conditions of the atmosphere outside. 

 The point is this: the humidity inside the egg-chamber must 

 be the same, whatever the conditions are outside. 



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