GENETICS OF REPRODUCTIVE PHYSIOLOGY 281 



possible that the primary genetic unit is a hormonal complex and that the hormones 

 as we know them are units of secretion or perhaps chemists' artifacts. In this regard, it 

 would be interesting to know the gonadotrophin content of the pigs of the slow-growing 

 strain, the ones that were not cystic. Were you able to determine this, Dr. Nalbandov ? 



Dr. Nalbandov has rightly emphasized the need to study each step of the repro- 

 ductive process separately, and I think this is particularly important because there are 

 many mutually inhibitory reactions in the various steps. I think it was by the elimina- 

 tion of the reproductive phases of brooding and incubation that it was possible to progress 

 so far with the genetics of egg production of chickens. With improved techniques of 

 the husbandry of the newborn, we may be able to study reproduction in mammals 

 uncomplicated by the process of lactation. 



Dr. Wright: I was interested in Dr. Nalbandov's correlation of large size and large 

 litters. Many years ago, I was working with 23 different inbred strains of guinea 

 pigs. They came to differ characteristically in each element of vigor: percentage 

 born alive, percentage raised of those born alive, regularity in producing litters, size 

 of litter, weight at birth, and gain to weaning. The weights and rates of gain were 

 strongly correlated, but for the most part the averages in these different elements of 

 vigor showed no correlation among the strains. There was consistency in the particular 

 combination of characters of each strain, year after year. One family would be very 

 high in regularity and low in size of litter, and we encountered almost every possible 

 combination with but one exception. There was a strong correlation between weight 

 and size of litter such as Dr. Nalbandov has found. 



Dr. Nalbandov: I find this very interesting, Dr. Wright. It is possible that only 

 these two hormones are responsible for body size and for litter size and that they are 

 linked. 



Dr. Wright: Yes. 



Dr. Nalbandov: Mr. Whitten asked about the hormonal content of pituitary 

 glands of noncystic animals. We had so few animals that the assays are questionable. 

 What data we have show that the content of gonadotrophic hormone of the hypophyses 

 in the small, slowly growing subrace was lower than it was in the rapidly growing 

 strain. 



Dr. Gowen : Have you examined mice of the strain Goodale established, as well 

 as MacArthur's? It has been our experience that the Goodale strain is difficult to 

 maintain in reproduction. Also, strains of mice differ widely in their reproductivity 

 over their life span. Some strains have large litters at first and soon play out. Other 

 lines have relatively uniform litters over long periods of time. Others show a curvi- 

 linear increase or decrease in litter size with aging of the parents. Others are so 

 fertile they breed every estrus. Hormonal variations probably contribute to these 

 differences. Do you have any data covering these possible interactions? 



Dr. Heston: We have that line of Goodale's, and my impression is that they have 

 large-sized litters when they do have them. However, many of them do not raise their 

 litters, and many of them are sterile. 



