418 J. A. MYERS 
pletely formed. Near the highest part of the nipple elevation 
in the new-born female appears a shallow pit, the first indication 
of the milk-pore and the beginning of the intra-epidermal part 
of the primary duct. As age advances, this pit deepens and 
enlarges until at five or six weeks it comes in contact with the 
lumen of the primary duct proper. Thereafter the system of 
ducts is in direct communication with the surface through the 
milk-pore. 
The walls of the ducts, especially the primary and secondary 
ducts, are quite thick at birth. The free half of each terminal 
enlargement is composed of a solid mass of epithelial cells. At 
birth some of the ducts toward the free end of a system present 
a wall composed of two definite layers of cells. After the first 
week all of the milk-ducts except the attached end of the pri- 
mary duct are surrounded by an inner layer of very compactly 
placed low columnar or cuboidal cells and an outer layer of 
irregularly shaped cells with no regular arrangement (figs. 8 
and 9). 
Ancel and Bouin (’09) found in rabbits as many as ten milk- 
ducts radiating from the nipple. Each duct in animals before 
the stage of puberty, however, does not exceed 2 mm. in length. 
The diameter of the gland reaches about 3 or 4 mm. The fact 
that there is a single duct leading from the nipple in the albino 
rat perhaps accounts for the much greater length and branching 
of the ducts in the prepuberty stage. Steinhaus (’93) found in 
young guinea-pigs that the glands are composed only of excretory 
ducts. Dulcert (’93) arrived at the same conclusion. They 
did not observe many mitotic figures. In a heifer of one year 
Dulcert (’93) reported only excretory canals and observed no 
mitotic figures. O’Donoghue (712) found in the deeper parts 
of the glands of Dasyurus a few structures of doubtful signifi- 
cance which may or may not represent mitotic figures, but he 
found that the actual formative growth of the gland does not 
appear to start until after ovulation. 
Langer (’51), Raubitschek (’04), and others have reported 
true alveoli in the mammary gland of human after birth and 
before puberty. Berka (’12), however, was unable to find true 
