STUDIES ON THE MAMMARY GLAND 207 
fetuses there are still systems of ducts at twenty days and six 
hours which show absolutely no trace of a lumen. In other 
glands of this stage the lumina are much larger than in the pre- 
ceding stages (fig. 5). When present, the lumina are better 
developed in the free ends of the system of ducts, i.e., in the ter- 
minal ducts and the ones from which they arise; however, quite 
frequently traces of lumina are observed in the primary and sec- 
ondary ducts. In no part of any system of glands observed is 
there a definitive lumen present. The walls of all are irregular, 
but have quite sharp boundaries. In no case are degenerating 
cells found within the lumen. 
Figures 4 and 5 show that the first indication of a lumen is the 
appearance of a few independent lacunae. In cross section of the 
ducts such lacunae are usually seen located near the center of the 
developing ducts; however, they are not uncommonly found near 
the periphery, at the central ends of the peripheral layer of cells. 
When traced longitudinally any individual lacuna is found to 
extend only a very short distance; but in serial sections other 
lacunae are found forming more or less definite rows extending 
along the ducts. In some glands the lacunae are present from 
the end-buds well into the primary ducts. 
The lacunae later increase in size and apparently flow together, 
thus forming the lumina found in some individuals of this stage 
(fig. 5, l) . The lumina at this stage are never continuous through- 
out the system of ducts. But a lumen may extend throughout a 
terminal duct, then with an interruption appear again in the 
tertiary or secondary ducts. 
Owing to individual variation, it is possible to find all of the 
above described developmental stages of lumina in twenty day 
and six hour fetuses. 
Several of the glands of this stage were stained with Dominici's 
combination stain. In blood vessels, the corium within the epi- 
thelial hood, the connective tissue immediately surrounding the 
ducts, and the ordinary connective tissue in the entire gland 
region were found various kinds of white blood cells including 
eosinophiles. In one gland a few lymphocytes were observed 
in the developing lumina of the ducts. None of the glands ex- 
