Minot.] 198 [March 5, 
thinking that senescence is a distinct phenomenon, from the gradual 
disappearance of the rejuvenation. The third factor, the weight of 
the cells, diminishes rapidly at first, then remaining nearly constant. 
The rapid changes occur in the earliest embryonic period, hence we 
conclude that in posi-embryonic growth the main factors are (1), the 
rate of multiplication, (2), the senescence. These therefore are the 
most important to us, since it is only post-embryonic growth which 
we can readily study. The growth of animals has been measured in 
very few cases. Of recent investigations I will refer especially to 
those of Semper! on Lymnaeus, which are not so generally known as 
they deserve. The observations upon man are more extensive and 
reliable. By far the most valuable and trustworthy are the statistics 
published by Gould, and the recently published paper by Dr Bow- 
ditch. The older observations of Quetelet are, as we know through 
Bowditch? and Paghiani’, inaccurate, because he tabulated only picked 
cases. This is to be especially noted because the conclusions of 
Quetelet are very generally repeated in text books, and even in the 
second edition of Foster’s Physiology*. 
It is probable that the rate of growth in weight increases for about 
half the period of growth, then remains approximately constant for 
a short time and finally slowly diminishes until it becomes zero. 
Whether in all animals the succession of changes is the same the 
future will decide. In man and some few animals it certainly is so. 
Of these phenomena, I offer the following interpretation: — at first 
the increment of senescence is less than the increment of the number 
of cells at each division, therefore the multiplication of the cells ob- 
tains the upper hand, and the rate of growth increases; soon how- 
ever the increment of senescence surpasses the increment of multipli- 
cation and thereby renders the rate of growth less, until at last the 
excess of senescence becomes so great that growth ceases; or at least 
becomes so insignificant, that it only serves to counterbalance the 
loss of cells. Now since the cells multiply in geometrical progres- 
sion, the senescence cannot increase in geometrical progression 
also, because then the curve of growth would be a straight line; 
1 Semper. Arbeiten des zool. zoot. Inst. Wirzburg, I. (1874), 137. 
2 Bowditch. Growthof children. Eighth Annual Report of the State Board of 
Health of Massachusetts. Boston, 1877. 
3 L. Paghiani. Sopra alcuni fattoir dello Triluppo umano Atti. real Acad. Sci. 
di Torino, x1, (Marzo, 1877). 
4 M. Foster. A Textbook of Physiology. 2d. Edition (1878), p. 556. 
