400 J. A. MYERS 
Twelve hours 
In female rats twelve hours after birth the epidermis covering 
the developing nipple appears slightly thinner than at birth. 
There is no noticeable change in the epithelial hood. The at- 
tached end of the primary duct is still without a lumen. The 
remaining ducts possess quite well-developed lumina and their 
walls are generally lined with two layers of epithelial cells, as 
described in the preceding stage. However, in some cases the 
outermost layer is almost entirely absent from a small part of 
the ducts. These parts with a single layer of cells constituting 
their epithelial lining are not confined to the terminal ducts only, 
but may be found in any of the other ducts except the primary 
duct. 
There is a considerable extravasation of red blood-cells in the 
stroma around the ducts in some specimens. A considerable 
number of such cells are present in the walls of the milk-ducts, 
while a few appear in the lumina. 
In addition to some red blood-cells in the lumina of the ducts, 
a few leucocytes are present. A somewhat homogeneous sub- 
stance is also present in some segments of the ducts. In no case 
have degenerating cells and nuclei been observed! in the lumina 
of the milk-ducts. The cells and other contents present exist 
in such small quantities that distention appears in no part of 
any system of ducts examined. Special fat stains failed to reveal 
any trace of fat in the walls or lumina of milk-ducts. In the 
fetus it was found (Myers, 717 a) that the lumina of the milk- 
ducts are formed by the process of rearrangement of the cells 
and not, as was formerly believed, by the degeneration of the 
central cells of the ducts. At birth and for some time after no 
degenerating cells have been observed in the lumina; moreover, 
the free borders of the cells immediately surrounding the lumina 
are quite sharply indicated. It therefore is evident that the 
substance contained in the lumina of the milk-ducts outside of 
normal cells is not a detritus, but is probably a secretion pro- 
duced by the cells surrounding the lumen. 
