STUDIES ON THE MAMMARY GLAND 399 
arrangement into layers (fig. 7). As the primary duct leaves 
the epithelial hood and passes into the tela subcutanea, nearly 
to its bifurcation, the lumen presents a very irregular outline, 
in some sections being absent entirely. The walls of the duct. 
are from three to four cells thick. 
The secondary ducts which result from the bifurcation of the 
primary duct possess lumina with more regular outlines. The 
cell walls appear somewhat thinner and there are indications 
of the cells beginning to arrange themselves into layers. From 
the secondary ducts to the ends of the terminal ducts there is, 
in most cases, a fairly definite layer of low columnar or cuboidal 
cells surrounding the lumen. External to this layer is a second 
layer of somewhat irregular-shaped cells which is present in most 
places, but may be partly or entirely lacking in some parts of the 
terminal ducts. The free ends of the terminal end-buds are 
composed of solid masses of epithelial cells. 
The milk-ducts are usually spread out so as to occupy a single 
plane (fig. 18) except in the case of the second inguinal gland 
(Myers, 716), where the limited space for the distribution of ducts 
forces them into two or more planes. Immediately surrounding 
all segments of the ducts are a few concentrically placed layers 
of connective-tissue which represents the future mantle layer. 
The irregularly arranged connective-tissue between the adjacent 
ducts and the mantle layer may be regarded as representing a part 
of the true stroma found in later stages. 
As previously shown (Myers, ’17 b), no trace of a mammary 
gland appears on the surface of the skin of new-born male rats. 
This is due to the fact that the epidermis in most cases presents 
no thickening in the region of the mammary gland. The skin, 
therefore, is as transparent over this region as in that immedi- 
ately surrounding the gland, so the color of the blood in the 
corium gives a uniform appearance to both areas. 
The end of the primary duct attached to the epidermis in the 
male is represented by a solid cord of cells. The free end of this 
duct and the secondary ducts possess lumina in slightly more 
advanced stages of development than in females of corresponding 
age. The remaining parts of the gland correspond closely in 
structure and development with the glands of the female. 
