406 J. A. MYERS 
(three or four deep) that have not yet arranged themselves into 
definite layers. As the primary duct is traced from the epithelial 
hood into the tela subcutanea, where it turns at right angles and 
courses parallel with the surface of the skin, the walls of the duct 
become thinner. The cells are here arranged in layers, rarely 
more than two layers present. The lumen has a well-defined 
outline which is occasionally interrupted by the budding off of 
short lateral buds. Many of these buds likewise possess lumina. 
The remainder of the ducts has the same general appearance in 
structure as at the end of one week. The epithelial walls are 
composed of two layers of cells (fig. 8). 
The contents of the milk-ducts have diminished in quantity 
since the earlier-described stages. 
Hair follicles are very numerous peripheral to the sulcus sur- 
rounding the nipple superficial to the epithelial projection, but 
they have in no specimen been observed medial to this groove. 
Three, four, five, six, and seven weeks 
From the end of the second to the beginning of the eighth week 
the growth of the mammary gland, as shown in an earlier work 
(Myers, 716), in the female albino rat is apparently only sufficient 
to keep pace with the general body growth. Cleared prepara- 
tions show that beyond question the ducts of the mammary gland 
grow in length and, moreover, send out numerous new collateral 
branches during this period. The nipple makes a gradual 
increase in height. The groove has approximately the same 
extent around the nipple as in rats of two weeks, but it becomes 
deeper as age advances. The projection of the epithelial hood 
retains its processes (fig. 4). The epithelium between the lumen 
of the intra-epidermal part of the primary duct and the lumen of 
the primary duct proper at the end of the second week is com- 
pletely broken down about the fifth or sixth week. The lumen of 
the primary duct proper therefore becomes directly continuous 
with that of the intra-epidermal part of the duct and communi- 
cates with the exterior through the milk-pore. The lumen of the 
intra-epidermal portion of the duct is through the seventh week 
