of thyroid activity was needed l)efore metabolic rate could he modified 

 hy genetic selection. 



The level of circulating protein bound iodine was not a satisfactory 

 measure of thyroid gland activity in the fowl (34) . 



In vivo changes in the Ii"^^ content of the thyroid gland were esti- 

 mated liy giving graded doses of pure L-thyroxine (37). The birds were 

 fed thiouracil to minimize reutilization of Ji^i from metabolized hor- 

 mone. This method was useful for group comparisons but not for indivi- 

 dual birds. One in vivo technique using I^^i, thiouracil, and replace- 

 ment thyroxine was effective in discriminating individual differences. 

 However, it was considered too complex for routine use. 



The technique of measuring, in vitro, the red blood cell uptake of 

 radioiodinated triiodothyronine was found to be inappropriate in 

 chickens. 



The relative potencies of L-thyroxine and of L-triiodothyronine in 

 reducing thyrotrophic hormone secretion were determined by the radio- 

 iodine method and by goiter-prevention assay (35, 36) . Although L-tri- 

 iodothyronine had been shown to be several times as effective as L- 

 thyroxine in mammals, this was not true in chicks. In fact, L-thyroxine 

 was somewhat more potent in chicks. 



Thyroprotein and thiouracil were fed to young White Leghorn 

 chickens to investigate the metabolic rate as measured by oxygen con- 

 sumption (33). Results depended largely upon the fasting period prior 

 to metabolic rate determinations. Metabolic rate was depressed by 

 thiouracil for 18 hours after the drug was withheld after which it re- 

 turned to normal. Metabolic rate was stimulated by thyroprotein only 

 a few hours after the drug was withheld. 



The relationship of metabolic rate to body size was investigated. It 

 was concluded that either the use of a regression adjustment for indivi- 

 dual animal data or the % power of body weight gave a reasonably 

 good correction for differences in body size. 



Yolk Cholesterol 



Egg yolk cholesterol was measured at the Maryland Agricultural 

 Experiment Station in a sample from the Cornell randombred control 

 population of Single Comb White Leghorns (16, 17, 18, 19, 20). Signifi- 

 cant differences among individual hens were found but no differences 

 due to sire or dam were detected. The cholesterol level was expressed 

 as mg. cholesterol per gram of yolk on a wet basis. Yolk cholesterol of 

 eggs laid in November was phenotypically correlated with egg weight 

 (0.19), specific gravity (0.27) and yolk cholesterol of eggs laid by the 

 same hens the following March (0.32) . A negative correlation was found 

 ])etween egg yolk cholesterol and yolk weight (-0.15). Yolk cholesterol 

 was not correlated with serum cholesterol, age at first egg, egg produc- 

 tion, body weight or albumen quality. Egg yolk cholesterol varied with 

 season. The concentration of cholesterol increased from 25.1 milligrams 

 per gram of yolk in August to 26.1 in February after which it decreased 

 to 25.3 in June. 



The feeding of crystalline cholesterol to Single Comb White Leg- 

 horns of a commercial strain resulted in an increase in yolk cholesterol 

 of almost 50%. The increase in serum cholesterol ranged from 30 to 



