416 J. A. MYERS 
called attention to such projections. Bresslau (’12) observed 
these projections developing in Talpa europaea, white mouse, 
Mus musculus, Mus rattus, and Mus decumanus, and concluded 
that they develop into the secondary sheath or pocket of the 
nipple. In the albino rat wax reconstructions have been used 
by me to show that the epithelial projections seen in sections 
form a continuous hood around the primary duct. The attached 
end of this hood lies immediately peripheral to the base of the 
nipple. In view of the fact that the present work shows that 
the sulcus is deepened through degeneration and desquamation 
of the attached end of this hood, we must conclude that a portion 
of the sulcus is formed at the expense of the hood. In other 
words, the lateral wall of the attached part of the hood becomes 
the lateral wall of the deep part of the sulcus, while the medial 
wall of the attached end of the hood becomes the medial wall 
of the deep part of the sulcus. At present I am unable to say 
whether the sulcus forming the pocket around the nipple con- 
tinues to deepen in the same manner during pregnancy and 
lactation until the free end of the hood is reached. 
Milk-ducts 
It will be recalled that the milk-ducts present several branches 
in female fetuses of about twenty days (Myers, ’17 a). A con- 
siderable growth and branching occurs between this stage and 
the time of birth (compare fig. 12, p. 223, vol. 22, Am. Jour. Anat., 
with figs. 3 to 6, inclusive, pp. 361-365, vol. 19, Am. Jour. Anat.). 
At the time of birth the lumina of the milk-ducts are not 
completely formed. The attached end of the primary duct for 
a short distance has no trace of a lumen. However, as one 
passes toward the deeper part of the duct one observes isolated 
lacunae. A little further along the duct one meets larger spaces 
which result from the flowing together of several lacunae (fig. 7). 
Finally, a slit-like lumen appears in the still deeper part of the 
duct, which extends through the secondary ducts. In the re- 
mainder of the ducts a fairly well-developed lumen is present. 
As age advances the lumen of the primary duct becomes com- 
