STUDIES ON THE MAMMARY GLAND 405 
superficial to the attached end of the epithelial hood, may be seen 
short strands of epithelial tissue which react to the stains differ- 
ently from the surrounding epithelium. Higher magnification 
shows that the cells in such strands are undergoing degeneration 
and are being cast out, thus deepening the groove. 
The epithelial projection of the hood has taken on a marked 
development since the fifth day. It still presents processes 
which are now more numerous on the inner side of the projection. 
Such processes are likewise present on the inner surface of the 
epidermis covering the nipple. 
As the layer of smooth muscle which lies immediately below the 
hair follicles reaches the neighborhood of the nipple, it turns 
toward the surface and enters the epithelial hood. Here many of 
the fibers take a course parallel with the primary duct and appar- 
ently enter into the formation of the sheath of the duct. Others 
soon take an oblique direction and apparently attach to the pro- 
cesses on the inner surface of the epithelial projection. Some 
muscle fibers, after extending into the nipple, take an oblique 
course and appear to attach to the processes on the inner aspect 
of the surface epidermis of the nipple. The function of the 
smooth muscle fibers will be dealt with in studies on the activity 
of the mammary gland. 
The lumen of the intra-epidermal part of the primary duct contain 
debris apparently in sufficient amount to distend the walls of this 
part of the duct. Near the milk-pore the wall is reduced to a 
rather thin structure. As the bottom is approached, the wall 
becomes thicker. It is here that desquamation is taking place at 
present. The cord of epithelial cells, which in all of the previous 
stages separated the lumen of the intra-epidermal part of the 
duct from the lumen of the primary duct proper, is at this age 
perforated by a very narrow canal. Thus for the first time a 
communication is established between the lumina of the system 
of milk-ducts and the exterior. 
The lumen of that part of the primary duct within the epithe- 
lial hood and nipple now has a fairly regular outline as far asthe 
region intervening between it and the luman of the intra- 
epidermal part of the duct. Here the walls consist of cells 
