BIRDS' EGGS 



249 



day) increases twenty-five fold — mainly by the rapid addition 

 of yolk. 



There is a central core of what is called " white yolk " 

 (really yellow), physically different from the " yellow yolk," 

 and there are zones of " white yolk " occurring at intervals 

 amongst the " yellow yolk." The difference between the 

 " white yolk " and the " yellow yolk " depends on the 

 nature of the minute " yolk-plates," which are laid down in 



wH.y y.y 



Fig. 38. — Diagrammatic section of egg. N., nucleus ; p.p., formative 

 protoplasm; v.m., vitelline membrane; CH., chalaza ; SH., shell; sh.m., 

 shell-membrane ; alb., albumen ; y.y., yellow yolk ; wh.y., white yolk ; 

 A.C., air-chamber. 



concentric layers, and the white yolk has more water in it 

 than the yellow yolk. It is a familiar fact that the yolk of a 

 hard-boiled egg shows alternating zones. On the top of the 

 central core of " white yolk " there is a clear drop of living 

 matter centred in a nucleus. This is the formative proto- 

 plasm of the ovum, out of which an embryo may be developed. 

 However the laid egg be turned about, the dilated egg-cell 

 (popularly called the yolk) turns so that the formative disc 

 is uppermost, and this is important because the movement 

 brings it nearest the brooding mother, the source of warmth. 

 Part of the yolk of the egg is of a fatty nature, and it is 

 interesting to notice that there is a correlation between the 

 fat content in the blood of fowls and the total egg records. 

 The correlation is positive for birds in a laying condition ; 



