422 J. A. MYERS 
three parts: first, the short excretory duct which extends from 
the milk-pore to a noticeable enlargement; second, the enlarge- 
ment or milk-sinus, and, third, the milk-duct or canal. He 
found the excretory duct lined with a stratified pavement epi- 
thelium, while the sinus and milk-ducts possess two epithelial 
layers. Benda (’94) found the epithelium near the attached 
parts composed of two layers of cells, while in the deeper and 
free parts of the ducts the epithelium was described as consisting 
of a single layer. Raubitschek (’04) found in a human female 
of seven days that the epithelium and the ducts is composed of 
two layers. Brouha (’05) described this epithelium as consisting 
of two layers throughout the entire system of ducts in virgin 
Vespertilio murinus, the rabbit, and the domestic cat. Schil 
(12) observed two distinct layers of cells around the ducts before 
the stage of puberty. O’Donoghue (’12) confirms Brouha’s 
finding in the domestic cat and writes concerning Dasyurus as 
follows: ‘‘As in the foetus, so in the adult Dasyurus, all the 
branching tubules of the mammary gland are lined by a double 
layer of cells, at any rate in the resting animal, and for some 
time after ovulation.”’ 
The fact that a few ducts were observed in the early postnatal 
stages of the albino rat with small areas covered with a single 
layer of epithelium probably has no significance as in later stages 
all ducts observed were lined with a double layer of cells. The 
outer layer of cells doubtless represents the so-called basket cells 
or myo-epithelial cells of the active mammary gland, while the 
inner layer represents the true glandular cells. 
True alveoli lined with a single early of cells have been re- 
ported in the glands of various animal species before puberty 
by several writers. Such alveoli or acini have probably been 
observed, as there is no reason to doubt that through individual 
variation a few acini might develop just as a mature Graafian 
follicle sometimes appears in the ovary of the human fetus or 
new-born. However, Schil (’12) concluded that during this 
period the mammary gland in the rabbit ordinarily presents 
no glandular acini. O’Donoghue (’12), after studying the glands 
of Dasyurus, made the following statement: ‘‘The true secre- 
