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CHAS. H. o’dONOGHUE. 
just after parturition (fig. 5) as it is just before (fig. 4). 
In the gland of the animal with newly born young (No. 
18), and also of the one with young a few hours old 
(No. 19) no sign of milk is to be found either in the 
alveoli or in the ducts, although there is a fair amount of a 
colostrum-like secretion present. 
The first appearance of true milk is in No. 20, where the 
young are twenty-four hours old, and even here it is confined 
to the aveoli and is not found in the main ducts. It is not until 
No. 21, that is, thirty-six hours after birth, that we find 
the gland and main ducts containing milk. The lining of 
the gland in this animal presents the appearance of a typical 
glandular epithelium in full activity, and the individual cells 
have still a more or less cubical form (fig. 17). The 
original two-layered condition of the tubules and acini has 
completely disappeared in the deeper parts of the gland and 
has given way to a single-layered one, and it is only in the 
central portions near the base of the teat that we find the 
tubules adjoining the main ducts with two layers of cells. 
The acini in the central portions, of the gland are already 
beginning to enlarge considerably, and the ducts also have 
increased in size (fig. 6). 
From this time onward the secretion becomes more and 
more pronounced, and the individual alveoli become increas- 
ingly distended, causing the epithelial cells lining them to 
be stretched and flattened to a remarkable extent. Fig. 8 
shows a part of the gland in No. 25, that is, four months 
after the birth of the young, when the milk-flow is at its 
height, and it will be seen that the increase in the size of the 
gland is enormous. Comparison, however, with the three 
preceding figures shows that this increase is entirely due to 
the huge distension of the alveoli of the gland by milk and 
not to an increase in the number of alveoli themselves. Such 
an enlargement necessitates a great stretching of the secretory 
epithelium, which under low magnification has the appearance 
of thin lines marking the outline of the alveoli (fig. 18). 
Contemporaneous with this great increase in the gland mass 
