lyiy] ANIMAL PRODUCTION. 767 



" The average weight of animals of both sexes increases, with a continuously 

 decreasing velocity, to a definite maximum, attained at about 91 weeks of age 

 in the male and at about 94 weeks of age in the female. After attaining this 

 maximum the [growth] curve descends rather steeply, the loss of weight being 

 much more rapid than the immediately preceding gain. This loss of weight is 

 the expression of senescence, and continues without interruption until the de- 

 cease of the last surviving animal. . . . 



" Tlie average duration of life (110 weeks in the male, 103 weeks in the 

 female) exceeds by a definite interval the period during which the weight of 

 the animals is increasing. The attainment of the average duration of life is 

 therefore preceded by a decided measure of senescence which must consequently 

 have affected the viability of the majority of the animals and played a leading 

 part in determining the mean duration of life." 



The rest of the paper presents similar data for two groups of males and two 

 of females born at later dates. Each group is compared with its predecessor 

 of the same sex. The intervals between the birth dates were, respectively, 4 

 and 20 months in the case of the males, and 24 and 15 months in the case 

 of the females. Having due regard to the probable errors of the differences, 

 these data " reveal a regularly diminishing velocity and absolute magnitude of 

 growth in each succeeding batch of animals." The differences were more 

 marked between the first and second than between the second and third groups 

 of each sex, and this is held to indicate an approach to a stable type. The 

 decrease in size and rate of growth is attributed to inbreeding, which was 

 unavoidable since new strains have not been introduced. 



XI. The grototh mid senescence of white mice fed upon pituitary {anterior 

 lobe) tissue, tethelin, egg lecithin, or cholesterol (pp. 393-426). — The growth 

 data presented in Studies III to VI (E. S. R., 35, pp. 865, 866) are extended to 

 cover the later periods of life so that the tables now published are complete 

 from the first weights at the age of 4 or 5 weeks to the time of death of the 

 last survivor. In the course of the four experiments ox pituitary, tethelin, 

 " lecithin " (ether extract of egg yolk precipitated once with acetone), and choles- 

 terol were each fed to a particular group of males and a particular group of 

 females that received in addition a normal mixed diet. A definite amount of 

 each substance was given daily throughout life except in the case of the 

 tethelin female group, where the administration was intermittent and ceased 

 before maturity. 



" In all the growth curves senescent loss of weight precedes by a definite 

 interval the average duration of life, and the terminal oscillations of the curve 

 reveal successively steeper descending limbs, showing that senescent loss of 

 tissue is proceeding with augmenting velocity as age increases. The position of 

 the maximum of the growth curve is, however, considerably affected by the 

 administration, and almost disappears in the curve of the tethelin males (which 

 received tethelin continuously) and in the curves of the lecithin-fed animals. 

 The hump of the normal curve is replaced in these cases by a long, flattened 

 plateau, gradually rising (tethelin males) or falling (lecithin males and 

 females) till the relatively rapid fall due to senescence supervenes. It is prob- 

 ably significant that the greatest flattening of the curve (i. e., slowing of the 

 rate of change of growth velocity) occurred in precisely those groups of animals 

 which displayed the greatest duration of life (tethelin males), or else in which 

 certain exceptional individuals displayed a remarkably enhanced duration of 

 life (lecithin males)." 



XII. The influence of pituitary gland (anterior lobe) tissue, tethelin, egg 

 lecithin, and cholesterol upon the duration of life of the white mome (pp. 427- 



