STUDIES ON THE MAMMARY GLAND 403 
in paraffin, and stained in other fat stains, proves fairly conclu- 
sively that they were carried by the razor from the fat in the 
connective-tissue stroma to the epithelial walls of the ducts. 
Leucocytes inside the lumina are very rare at this stage. A few 
normal cells with nuclei resembling those of the epithelial wall are 
‘present in the lumina. In a few instances normal nuclei with 
apparently no accompanying cytoplasm were observed in the 
lumina. Such elements are slightly more numerous toward the 
free ends of a system of ducts. In only one or two instances have 
the cells within the lumina shown any signs of degeneration. 
Dominici’s combination stain reveals some leucocytes in the 
stroma outside the ducts and in the epithelial hood, but no infil- 
tration of such cells in the stroma around the ducts has been 
observed. There apparently is no extravasation of red blood- 
cells as was observed in rats of one day. 
End of first week. 
At the end of the first week the nipple is more elevated, the 
lumen of the intra-epidermal part of the primary duct and the 
projection of the epithelial hood extends deeper into the corium. 
The lumen of this part of the primary duct is now nearly filled 
with debris in which appear some desquamated epithelial cells. 
The processes on the epithelial projection of the hood are very 
conspicuous. 
The milk-ducts in most places possess two layers of epithelium. 
The inner layer is formed of very compact epithelial cells of the 
low columnar or cuboid type. The cells of the outer layer are 
less compact and in fact may be absent in some places. They are 
irregular in size and shape. The amount and quality of the 
secretion in the lumina is approximately the same as at four and 
five days. 
At this stage the primary duct of the male rat is attached to the 
epidermis, there being no nipple at any’stage. Its attached end 
does not possess a lumen. In some eases there is a very slight 
excavation on the surface of the skin immediately over the at- 
tached end of the primary duct which resembles the developing 
