16 HELEN DEAN KING 



as shown by data in various tables, these rats attained matur- 

 ity earlier than did those of preceding generations, their fer- 

 tility was greater, the reproductive period longer, mortality 

 during early life much less, and the average life span pro- 

 longed. There is no evidence, however, to warrant an assump- 

 tion that rate and extent of body growth in these rats were 

 precocious. Possibly growth was approaching the optimum 

 for the race when individuals are maintained under environ- 

 mental and nutritive conditions favorable for the full ex- 

 pression of growth activators without overstimulating them. 



Although intrauterine factors doubtless play a major role 

 in determining the growth of fetal young, rate and extent of 

 postnatal growth, if environmental and nutritive conditions 

 are uniform, must depend mainly upon growth potentialities 

 which have their basis in the genetic constitution of the indi- 

 vidual. It has been shown for the guinea pig (Bessesen and 

 Carlson, '23), the rabbit (Kopec, '26), the mouse (Kopec, '29) 

 and the rat (Dunn, '08; King, '16) that the early postnatal 

 growth of individuals of the same litter is usually in the 

 order of their birth weights. Individuals that are large at 

 birth tend to grow more rapidly and to attain a larger size 

 when adult than do those with a low birth weight. 



Eats much below normal weight at birth, so-called runts, 

 never attain the body size of other members of the litter even 

 when reared under nutritive conditions that normally induce 

 rapid growth (King, '16). Kunts, apparently, are individuals 

 in which growth processes have been retarded by some in- 

 herent factor. Their inability to grow at a normal rate cannot 

 be due merely to nutritional handicaps during fetal life, since 

 restriction of growth by malnutrition does not deprive normal 

 rats of the power to grow when the diet is again adequate, as 

 McCay, Crowell and Maynard ('35) have shown. 



Another type of individual in which body size during adult 

 life is as much above the norm as that of the runt is below 

 it has been found in this strain of captive gray rats. Such 

 rats are not noticeably large at birth, and they grow during 

 early life at about the same rate as other individuals of the 

 same sex in the litter. To show the course of body growth in 



