180 C. M. McCay 



money they are seldom repeated. Our own experience has 

 made us very dubious about the adequacy of conclusions 

 based upon a single study even when these are certified by 

 the statistician. This is the reason we repeated our study of 

 retarded growth three times in the course of about fifteen 

 years. 



In spite of these weaknesses of the rat for research upon 

 old age it has great merit. The female of this species outlives 

 the male as she does in the case of man, so this great basic 

 problem of so much interest to the sociologist and economist 

 is subject to study using the white rat. The great problem of 

 extension of life-span by the retardation of growth is easily 

 studied with the rat. Only a beginning has been made in 

 determining why growth retardation should affect chronic 

 diseases and even the development of malignant growth. 

 Unfortunately, in the whole field of cancer research no one 

 has discovered a closer relationship between nutrition and 

 malignancy than this rather indirect one in which partial 

 fasting inhibits the onset or growth of a tumour. Either mice 

 or rats are useful in this area of investigation (Visscher et al., 

 1942). 



In some respects white rats afford rather unique oppor- 

 tunities. They lend themselves readily to parabiotic unions 

 in which two individuals resemble externally Siamese twins 

 and share in common many substances exchanged through 

 the humoral fluids such as potassium iodide, lactose, strych- 

 nine, methylene blue, thyroxine, insulin and sex hormones. 

 Since little is known about the ageing of such pairs of animals 

 or even the span of life, we prepared a couple of pairs in the 

 summer of 1953 and they have survived about a year. No 

 one has reported such operations to unite young and old. 



Rats have been extensively used in psychology research 

 but very little has been done in relation to age. Likewise, in 

 studying the effect of age of the mother upon the quality of 

 progeny, the rat affords unique possibilities. In our own 

 laboratory, after observing some years ago that the long- 

 lived rats came from a few mothers, a study was made to 



I 



