408 J. A. MYERS 
still engorged with debris. That part of the debris which occu- 
pies the milk-pore and is in contact with the exterior apparently 
becomes solid, thus forming a plug. In some specimens this plug 
has been removed from the milk-pore during the process of sec- 
tioning and lies as a solid body outside of, and independent of the 
milk-pore (fig. 4). Owing to the presence of such a plug, the 
contents formed in the lumen of the intra-epidermal part of the 
primary duct are unable to escape. 
The contents of the milk-ducts gradually diminish in quantity 
after about the second or third week. However, considerable 
debris is present in the lumina at the beginning of the eighth 
week. 
In all specimens examined the smooth muscle fibers of the 
nipple have the same arrangement and distribution as in the 
earlier described stages. 
The terminal end-buds of the ducts are prominent in the above- 
described stages. The lumen of the terminal duct gradually 
increases in size until it reaches a point near the middle part of 
the end-bud (fig. 11). Here the lumen ends abruptly, and the 
remainder of the end-bud is represented by a solid mass of 
epidermal cells. 
It has been pointed out (Myers, ’17 b) that the milk-ducts of 
the male fail to proliferate as rapidly as those of the female after 
about the fourth or fifth week. This difference together with the 
absence of the nipple and epithelial hood constitute the only 
characteristics that serve to distinguish the glands of the male 
from those of the female from the second to the beginning of the 
eighth week. As far as the structure of the remaining parts of the 
gland is concerned, one is unable to distinguish between the sexes. 
Figure 16 shows the milk-pore on the surface of the skin of a male 
rat of six weeks. The intra-epidermal part of the primary duct 
contains a shallow lumen. A solid mass of epithelial cells sepa- 
rates this lumen from that of the primary duct proper. 
The connective-tissue sheath or mantle layer surrounding the 
milk-ducts becomes somewhat thicker as age advances. This 
sheath is quite thick around the primary and secondary ducts, 
but when traced toward the free end of a system of ducts gradu- 
