8 
Proceedings of the Royal Society of Edinburgh. [Sess. 
prevented this, although it extracted a small quantity of milk, as shown by 
its cloudy appearance at the end of twelve hours. It is to be understood, 
then, that the mammae were weighed out of formalin. 
The basis of comparison between the meat- and the bread-and-milk - 
f ed rats is the percentage weight of the mammary tissue to that of the 
mother. 
By a reference to the tables below it will be seen that there are 
wide individual variations in this percentage among the bread-and-milk 
animals. 
Table giving the Lactation History of Eight Rats fed on an 
exclusive Ox-Flesh Dietary. 
No. of days 
on meat 
before 
parturition. 
Weight of 
animal at 
death. 
Weight of 
mammae. 
Per cent. 
Time in 
lactation 
when killed. 
Number 
of young. 
Weight 
of young. 
30 
grms. 
180 
grms. 
13 
7-2 
1st day 
10 
grms. 
50 
9 
140 
8*5 
6 
1st day 
10 
49 
27 
230 
19-5 
8-4 
1st day 
25 
150 
13 
8-6 
7 th day 
5 
48 
12 
115 
10*5 
9T 
20th day 
6 
120 
15 
130 
12 
9-2 
21st day 
7 
185 
21 
110 
10T 
9T 
21st day 
6 
110 
24 
140 
11-5 
8*2 
22nd day 
4 
82 
Average percentage = 8'2. 
This is explained by the different times during the course of lactation 
at which the animals were killed, and also by the varying numbers of 
young which they suckled. Thus there is a fairly uniform rise in the 
percentage from the first up to the twenty-first day, after which there is 
again a fall ; and taking animals killed at the same lactation period, the 
percentage is higher in those which nursed the larger number of young. 
We may therefore take it that the mammary gland in the rat is most 
actively functioning about the twenty-first day of lactation, and after this, 
as the young begin to feed themselves, it undergoes atrophy. 
It will be seen that the average percentage weight of the mammae of all 
the bread-and-milk animals is 9'6, while that of the meat-fed ones is only 
