GENETICS OF REPRODUCTIVE PHYSIOLOGY 271 



this hormone is known to act directly on the end organ. Size of comb of chickens is an 

 example of such a simple relationship. More often the pathway is indirect and in- 

 volves several hormones. Examples of the latter type include the whole reproductive 

 process, growth, lactation, and, in fact, most of the phenomena governed by hormones. 

 To complicate the picture still further, it is now known that in addition to the endocrine 

 system the nervous system is equally important in the control of phenomena which 

 earlier were thought to be directed by the endocrine system alone. In spite of these 

 difficulties, valuable information can be obtained by using the various techniques 

 which provide a guide to rates of hormonal secretion. 



ESTIMATES OF RATES OF HORMONAL SECRETION 



Bioassays of glands yield information on the concentration of hormones in them. 

 This method has the disadvantage that the gland-giving animals must be sacrificed or 

 mutilated. Furthermore, endocrinologists are not certain whether such bioassays 

 estimate the rate at which hormones are secreted, the rate at which they are stored, or 

 whether they simply indicate the amount of hormone remaining in the gland after its 

 physiologically effective quantity has been released into the blood stream. Obviously 

 the value of such bioassays hinges on which of these three alternatives is being estimated. 

 While this question has not been resolved to the satisfaction of all endocrinologists, 

 there is much evidence which shows that such assays do indeed estimate the rates at 

 which hormones are being secreted. 930 



It would be most desirable to obtain information on the amounts of the various 

 hormones circulating in the blood, but no good and easy methods are available to make 

 such estimates either by chemical or biological means. This is because hormones, 

 once they enter the blood, are destroyed or inactivated within minutes or, at most, 

 within a very few hours. Because of this difficulty, and because the concentration of 

 hormones in the peripheral circulation is extremely low, it becomes necessary to use 

 special techniques which sample the blood as it leaves the more accessible glands or 

 end organs. These techniques are cumbersome and difficult and have only limited 

 application in the study of populations of the size required for adequate genetic analysis. 



In addition to these direct methods of obtaining information on the rates at which 

 certain hormones are secreted, various methods permit indirect estimation of rates of 

 hormone production. To obtain such estimates, a hormone is allowed to act on its 

 target organ and the degree of stimulation of the target organ is used as an index of the 

 rate of secretion of the hormone. Differences in rates of ovulation in females belonging 

 to different genetic strains provide good examples of the variable rates of hormonal 

 secretion by pituitary glands. Rates of ovulation, expressed either in numbers of eggs 

 recovered from the female ductal system, or in numbers of corpora lutea found in 

 the ovaries, have been used to demonstrate pronounced genetic differences between 

 strains of domestic and laboratory animals. Litter size is an indirect measure of rate 



