STUDIES ON THE MAMMARY GLAND 401 
The stroma very distinctly presents the anlages of its two 
parts, the mantle layer immediately surrounding the ducts and 
the true stroma intervening between the ducts of a system of 
ducts. The true stroma is rather compact and contains many 
developing fat cells located toward the free ends of the ducts. 
Twenty-four hours 
_ Twenty-four hours after birth the developing nipple presents 
about the same appearance as in the preceding stage. The 
epithelial projection of the hood now possesses a few very short 
processes, some of which are located on the inner surface and 
extend toward the primary duct, while others are on the outer 
surface projecting away from the hood (fig. 2). Such processes 
are formed by a thickening of the stratum germinativum. In 
some cases the cells of this layer have only elongated, while in 
others at least two layers of cells enter into the formation of the 
processes. 
The contents of the lumina are approximately the same in 
quality and quantity as in the last-described stage. In the walls 
of the ducts just outside the lumen an occasional vacuole is seen. 
In the inguinal region laterodorsal to the inguinal and abdomi- 
nal glands a long mass of fat is beginning to appear in the tela 
subcutanea. This mass extends from the lateral surface of the 
abdomen just anterior to the ilium to a position posterolateral 
to the vagina. In the remainder of the present paper this mass 
of fat will be designated as the inguinal fat pad. Adipose tissue 
is also developing rapidly laterodorsal to the second and third 
thoracic glands and cephalic to the first thoracic gland, but it 
does not present such a definite mass as in the inguinal region. 
Four and five days 
In female rats of four and five days the nipple is somewhat 
more elevated above the surface of the surrounding skin than at 
the end of the first day of life (figs. 3 and 12). The nipple is now 
represented by a somewhat oblong elevation (fig. 12). Starting 
at a point slightly posterior to the first thoracic nipple and passing 
