Minot.] 1 96 [March 5, 
described. This law applies I believe to all the Metazoa, with the 
exception of the sponges. On another occasion I hope to show that 
similar alterations of size play the part of an important factor in the 
development of epithelium. 
6. The same cell may vary in weight. Such variations oceur (1), 
through alterations produced by the normal functional activity of the 
cells: such alterations have for instance been observed by Heid- 
enhain, Kihne, and many others in the cells of the salivary and 
pancreatic glands; (2), through changes in the non-protoplasmatic por- 
tions of the cells, as when water is lost or accumulated by the tissues, 
or when the fat varies. These variations are very great, yet by con- 
fining our attention to the average weights, we conclude that the 
cells diminish rapidly in size during the segmentation of the yolk, and 
afterwards change but little if at all—thus murine off two distinct 
periods: the first short, the second long. 
Our previcus formula was for the number of cells entering into the 
composition of the body at a given moment of time. To determine 
the weight of the body, the total number of eells must be multiplied 
by their average weight. Let y be the unit of weight and wy the 
weight of a single cell. Now, this weight is not constant but varies 
as a function of the time, hence we must multiply the number of cells 
into :f (w) y. Therefore: — 
Cis 
(2b"" — az) - fw) y 
in which 
x =interval between impregnation and first division, or the 
unit of time. 
c= coefficient of senescence, which determines the frequency 
of the divisions. 
z = number of cells originally = one egg-cell. 
b= ratio of multiplication. 
n= number of cell generations. 
a == number of cells lost. 
y = unit of weight. 
wy = weight of single cell. 
We now have an equation for determining the curve of growth as 
indicated by the weight of the body. This equation cannot be said 
to have any mathematical value, because of all the quantities we 
know exactly and certainly only the time z and the number of cells 
(z=1) when growth begins. Nevertheless I have constructed the 
