io8 



THE NEXT GENERATION 



sh m 



and as it thickens, nature packs into the shell a mass of albu- 

 men the white of the egg. This albumen does two things : 



a. It supplier moisture for the developing cell. 



b. It provides nourishment for the growing individual. 

 Moreover, the shell itself does two things : 



a. It is of such a texture that it keeps the moisture of the 

 albumen from evaporating even after the egg has been laid. 



b. It is so porous 

 a ^ that air gets into it for 

 the occupant to use 

 while it develops. 1 



With these facts in 

 mind, the natural ques- 

 tion is, Why are not 

 all animals ourselves 

 among the rest sup- 

 plied with eggshell? 

 and then hatched out 

 when the time comes ? 

 The answer is that 

 when the growing ani- 

 mal is to be small, it is comparatively easy for nature to 

 arrange to have it spend its first stage of growing within an 

 eggshell, but when an animal is to be so large that it must 

 develop for months instead of weeks before it is ready for 

 the outside world, then nature has to make some other ar- 

 rangement ; that is, no egg can be planned for big enough 



DIAGRAM OF BIRD'S EGG 



yk, yolk , /, germ disk ; alb, white, or albumen ; ch, 



thickened albumen which holds yolk in position ; 



a, air space ; sh. m, two shell membranes ; s/i, shell. 



(From Parker and Haswell) 



1 After an egg has been fertilized and laid, it must be kept at a definite 

 temperature for a definite length of time. Hens and birds keep their eggs 

 warm enough by sitting on them. Incubators have to supply the same 

 temperature, else the eggs will not hatch. Each kind of egg has its own 

 time limit for hatching. 



