INCUBATION 239 



4. A protective covering which is composed of a double mem- 

 brane within a hard shell. 



The germ may be seen, when the egg is broken, as a little white 

 speck on the yolk, and always on the upper side of the yolk, which 

 position it keeps because the yolk is suspended in the white by two 

 albuminous strings, and in whatever position the egg may lie, the 

 yolk turns, bringing the germ to the upper side. 



Note. An egg as described may be produced by the female bird without 

 association with the male. In the ordinary natural course the female on arriv- 

 ing at maturity (or at the breeding season) produces eggs which are complete 

 for commercial purposes and also, as far as her contribution to the egg goes, 

 for breeding purposes ; but the egg will not hatch unless the germ furnished 

 by the female has been fertilized by union with the sperm contributed by the 

 male at the proper stage of its development, nor will the germ thus fertilized 

 produce a creature of sufficient vitality for normal development if the germinal 

 elements contributed by the parents are lacking in vitality. Just how far a 

 superabundance of vitality contributed by one parent may compensate for a de- 

 ficiency in vitality in the contribution of the other is not known. That there is 

 a tendency to equalization is often apparent, yet it is just as evident that there 

 must be a certain degree of initial vitality in an element before it can unite 

 with its opposite sexual element for the production of a new organism. This is 

 illustrated best in the case of those hens of great laying capacity which produce 

 few or no chicks, their eggs rarely becoming fertile even with every oppor- 

 tunity to do so. The fact that a hen can produce, in extraordinary numbers, 

 eggs each of which apparently furnishes the material for a chick, though the 

 accompanying germ lacks the vitality which would enable it under proper con- 

 ditions to utilize that material, indicates that capacity to transmit vitality is more 

 restricted than capacity to produce material for the building of new organisms. 

 Of like significance in this connection is the fact that, though the male's contri- 

 bution to the egg is but a minute quantity of sperm, the capacity of the average 

 male to " strongly fertilize " eggs is plainly limited. These points are considered 

 more fully in the chapters relating to breeding. Mention is made of them here 

 to show that, in the nature of the case, the ordinary lot of eggs used for incu- 

 bation is unlikely to be high in " hatchability," — which fact must be given due 

 consideration in every effort to estimate causes of unsatisfactory hatches. 



A fertile egg. Technically, a fertile egg is an egg which has 

 fertilized germs possessed of sufficient vitality to develop so far 

 that development can be seen through the shell when the egg, after 

 having been incubated for a time, is tested by being held before 

 a light in the usual way. Fertility cannot be determined without 

 incubation. The amount of incubation necessary to show whether 



