238 



FOUNDATIONS OF BIOLOGY 



whether the developing embryo is in the main dependent upon 

 food stored in the cytoplasm of the egg itself, or upon some 

 outside source, such as the sea water in which it floats, or the 

 tissues of the parent. The first case is well illustrated by a 

 Bird's egg in which the so-called YOLK is the egg cell proper, 

 hugely distended by stored food, and surrounded by nutritive 



a, 



alb 



FIG. 128. Diagram of the egg of the domestic Fowl, before incubation, a, air 

 space between two layers of shell membrane; alb, ch, dense albumin (chalaza); alb', 

 more fluid albumin (white of egg) ; bl, point of cytoplasmic concentration from which 

 embryo arises (blastoderm); sh, shell; shm, shell membrane. (After Marshall.) 



and protective envelopes consisting of the 'white of the egg,' 

 shell membranes, and shell which are formed by secretions 

 from the walls of the oviduct during the passage of the egg to 

 the exterior. On the other hand the eggs of Mammals, for 

 instance of the Rabbit and Man, are very small the human 

 egg being less than 1/1 25th of an inch in diameter since 

 their essentially parasitic method of development in the uterus 

 renders superfluous the storage of any considerable amount 

 of food material in the egg cytoplasm. (Figs. 127-129.) 



