410 J. A. MYERS 
The nipple has enlarged somewhat and now at ten weeks more 
closely approaches the form of the nipple of the adult female rat. 
The groove at the base of the nipple has deepened apparently by 
the process of desquamation. In some individuals the tissue at 
the base of the nipple caudal to the first thoracic, cephalic to the 
second inguinal and lateral to the remaining nipples which in 
earlier stages connected the base of the nipple with the adjacent 
skin without the interruption of a furrow has now broken down 
soas toforma groove. This groove therefore extends completely 
around the base of the nipple. In fresh preparations the basal 
half of the nipple rests in the pocket formed by the groove. Such 
changes have not yet occurred, however, in all rats of nine and 
ten weeks of age (fig. 17). This furrow lies immediately super- 
ficial to the projection of the epithelial hood into which its cavity 
slightly extends. From the bottom of the furrow in some speci- 
mens may be seen the degenerating strands which were described 
in animals of two weeks (fig. 5). This furrow may contain debris 
which reacts to stains and appears under high magnification 
similar to the contents of the lumen of the intra-epidermal part 
of the primary duct. 
The intra-epidermal part of the primary duct is still engorged 
with debris. Its walls are lined with stratified squamous epi- 
thelium of approximately the same thickness as that covering the 
surface of the nipple. At the end of the intra-epidermal part of 
the duct there is a gradual transition from the stratified squamous 
epithelium of this part of the duct to the epithelium of the pri- 
mary duct proper. The walls. of this part of the primary duct 
are composed of two or three layers of cells. The innermost 
layer is composed of low columnar cells, while the remaining 
layers are composed of cells of more irregular form. As the pri- 
mary duct leaves the nipple and enters the deeper layers of the 
skin, its epithelium is arranged in two layers. The inner layer is 
composed of low columnar or high cuboidal cells containing 
elongated nuclei with their axes perpendicular to the lumen of the 
duct. The cells of the outer layer are somewhat irregular in size, 
shape, and position (fig. 9). In a longitudinal section of the pri- 
mary duct, as it courses parallel with the surface of the skin, 
