428 J. A. MYERS 
The nipple elevation is oblong in shape and very slightly 
elevated at the fourth or fifth day. At the end of the second 
week it is still oblong, but much more elevated. After the second 
week its growth is a gradual one until the time of puberty, when 
it takes on the size and shape of the adult virgin nipple. 
2. The sulcus around the nipple in the new-born female rat 
is apparently a remnant of the prenatal mammary pit. During 
the early days of postnatal life this sulcus disappears, but on 
the fourth or fifth day an apparently new depression makes its 
appearance. This depression deepens so that by the tenth week 
there may be developed a definite pocket in which the basal part 
of the nipple is located. 
3. The epithelial ingrowth (hood) at birth presents a smooth 
and quite regular outline. Two or three days after birth, how- 
ever, the stratum germinativum becomes somewhat thickened 
in some places, thus forming short processes. Such processes 
appear on both the outer and inner surfaces of the hood through- 
out the stages studied. The central portion of the proximal part 
of the hood apparently degenerates, thus deepening the sulcus 
around the base of the nipple. 
4. Hair follicles are numerous in the integument peripheral’ 
to the hood and sulcus, but have not been observed nearer the 
nipple. 
5. Near the highest part of the nipple there appears, a few 
days after birth, a slight excavation—the developing milk-pore, 
communicating with the intra-epidermal part of the primary 
duct. At two weeks there is a slight connection between this 
cavity and the lumen of the primary duct. About the sixth 
week a complete connection is established with the primary 
duct. When the nipple becomes cone-shaped, the milk-pore 
appears near the apex. 
6. The lumina of the milk ducts are apparently formed by the 
process of rearrangement of the cells. The lumen of the primary 
duct is not completely formed until after the second week. The 
other ducts possess fairly well-developed lumina at the time of 
birth. 
