STUDIES ON THE MAMMARY GLAND 411 
numerous processes may be seen some of which have lumina 
opening into the duct (fig. 6). These processes represent the 
collateral ducts. Some possess lumina and are lined with the 
same kind of epithelium as the primary duct, others are merely in 
the bud stage of formation and consequently have no lumina. 
Longitudinal sections of other ducts, as the secondary, tertiary, 
etc., likewise show numerous collateral ducts in various stages of 
formation. As the collateral ducts bud from the main ducts they 
carry with them a thin sheath of the connective tissue surround- 
ing the main duct. Mitotic figures are numerous in the epithelial 
cells in these stages. 
At this stage very little more than the ducts or excretory part 
of the gland is developed. However, at the end of some of the 
collateral ducts which branch from the secondary ducts one may 
observe several outpouchings which somewhat resemble a small 
cluster of grapes. Microscopic sections show that such out- 
pouchings possess lumina which are surrounded by a layer of 
cuboidal cells, external to which may be seen an occasional some- 
what flattened nucleus (fig. 10). These outpouchings may con- 
tain a slight amount of secretion, but they can hardly be called 
alveoli in the true sense of the word. True lobules are not yet 
present, although the above-mentioned clusters are doubtless an 
early indication of lobulation. The ramifications of ducts have 
become so numerous that they no longer occupy a single plane 
(fig. 19) as in the earlier stage (fig. 18), but form a rather dense 
arborization which in places reaches a thickness of 0.8 mm. 
No true milk sinus, such as has been observed by various 
authors in several animal forms, has appeared in any of the 
described stages of the rat. 
Elastic fibers are prominent in the mantle layer of the primary 
and secondary ducts. They are present in this layer of all the 
larger ducts, but tend to decrease in number as one passes toward 
the free end of a system of ducts. No elastic fibers have been 
observed in the sheath of the terminal ducts. The connective- 
tissue stroma is similar to that described in earlier stages except 
that it contains a greater amount of adipose tissue (fig. 19). The 
inguinal fat pad is so extensive that a thin layer of it now extends 
