GROWTH-CHANGES IN MAMMARY APPARATUS OF DASYURUS. 201 
The Mammary Glands. 
It is in the mammary glands, however, that the most 
marked changes occur during this period. The growth is 
manifested by an increase in the ramifications of the tubules, 
an increase in the size and activity of the individual cells 
lining them, and the formation of secretory acini. 
The tubules become more and more branched and give rise 
to numerous solid bud-like outgrowths, which appear in 
section as small masses of cells on the sides of the tubules, 
and in which mitotic figures are frequently to be found 
(fig. 15). Later the lumena of the tubules extend into 
these bud- like projections, the cells of which become 
arranged round the extension in a double layer. In this 
condition the outgrowths constitute the first stage of the 
alveoli. After these primitive alveoli have been formed in 
considerable numbers their lumena expand, and the double 
layer of cells lining them becomes reduced to a single layer 
of somewhat cubical cells. These structures are now the 
fully formed secretory alveoli (fig. 16). The larger part of 
this growth takes place prior to the attachment of the 
embryo to the uterine wall (stage No. 11). The cells of the 
inner layer increase very rapidly during the formation of 
the alveolus, and in consequence the outer layer becomes 
somewhat discontinuous, and its individual cells become 
elongated and flattened. It was suggested by Kolossow (30), 
that these cells become transformed into typical smooth 
muscle-fibres at the end of the growth of the gland, in 
which case we should have muscular cells derived from the 
ectoderm. I agree with Bronha (loc. cit.) that this does not 
appear to be the case, although perhaps it may be possible 
that they have a contractile function, as was suggested by 
Benda (loc. cit.). 
Bizzozero and Yassale (7) pointed out that the epithelial 
proliferations during the growth of the mammary gland in 
pregnancy resulted from the mitotic divisions of the original 
