INTRODUCTORY 27 



l)ird, and outside this food-yolk there is deposited a quantity of 

 " white " of egg, and finally, the whole mass is enclosed within the 

 shell. 



Although certain stages of development are passed before the egg 

 is laid, this development soon ceases, and is only resumed as a con- 

 sequence of the heat generated by the body of the brooding bird. 



At hatching, the young birds are in many cases extremely active, 

 following their parents and feeding themselves soon after leaving 

 the shell ; in other species, on the other hand, the young leave the 

 shell in a singularly helpless condition — blind and naked — and these 

 have to be very carefully attended by the parents until they are strong 

 enough to fly and fend for themselves. 



