276 THE BIOLOGY OF BIRDS 



The shell usually shows two layers — an inner or mamillary 

 which is always without pigment except in the Anis (Croto- 

 phaga), and an outer or spongy which may or may not be 

 coloured. Outside this there is often a pellicle or skin- 

 cuticle. As has been mentioned, the shell is porous and 

 there is an indispensable interchange of gases between the 

 developing embryo and the outside world. In at least some 

 pelicans and penguins the shell is still flexible when the egg 

 is laid ; but this simply means a certain prematurity in laying, 

 and need not be connected with the flexibility of the shell in 

 some Reptiles. In all birds the shell passes through a 

 flexible phase. 



Why should Eggs go Bad ? — It does not seem so easy as 

 one would think to answer this simple question. Out of 

 2520 fresh eggs examined by Philip B. Hadley and Dorothy 

 W. Caldwell (191 6) there was bacterial infection in the yolk 

 of 8*7 per cent. None of the 1 1 1 whites examined showed 

 any infection. The bacterial types were cocci, bacilli, and 

 spirilla, but it did not appear that they were of much im- 

 portance. They probably infected the eggs in the ovary, 

 having escaped from the intestine into the portal circulation. 

 According to the investigators this primary infection plays 

 no role in bringing about the decomposition of eggs nor in 

 causing embryo-mortahty. Why, then, do eggs go bad ? 

 For the factors determining decomposition secondary 

 infections must be looked to, and we suppose that the 

 microbes must either enter through the pores of the shell or 

 must be included about the shell-membrane and shell while 

 these are being formed in the lower part of the oviduct of 

 the hen. The use of " egg-glass," lime-water, and the like 

 for preserving eggs is to close up the pores of the shell, 

 excluding both microbes and air. 



Coloration, — Droplets of pigment are secreted from the 

 lower part of the wall of the oviduct, and these stain the egg- 

 shell before it has finally set. The staining may be diff^use, or 

 well-defined spots may be fixed when the egg remains for a 

 time stationary. As the egg may move onwards before the 

 secreted pigment is fixed— before the paint is dry- — there 



