GROWTH-CHANGES IN MAMMARY APPARATUS OF DASYURUS. 195 
and gives off a few short hollow branches. These latter are 
the mammary tubules. They are lined by a double-layered 
epithelium, the inner layer of which consists of somewhat 
cubical cells (flatter, however, than the cells of the corre- 
sponding layer in the adult) and the outer layer of more 
flattened cells. 
At a slightly later stage, in a pouch young twenty weeks 
after birth, the six strongly built hairs emerging from the 
teat anlage are still readily distinguishable. In section the 
hair is seen running into the primary outgrowth, and from 
the wall of this latter comes off the solid secondary outgrowth, 
which becomes hollow and branched in its deeper part to 
form the mammary tubule (fig. 1). The tubules have 
increased slightly in size and in numbers, and the lumen, at 
first confined to their distal extremities, has extended a 
little way along the solid cord towards the primary sprout. 
The wall of these tubules is in the characteristic two-layered 
condition, the cells of the inner layer being somewhat more 
cubical than those of the outer (fig. 11). From the upper 
part of the hair-follicle, just below the origin of the secondary 
outgrowth, another bud (the tertiary outgrowth) is originating 
which will give rise to the sebaceous gland. 
Each of the hair-follicles opening into the teat-pocket gives 
off one secondary outgrowth, and as there are six of these 
strong hairs, there are also six mammary gland anlagen. In 
the adult Dasyurus we find that each teat has opening on it 
six main milk-ducts, and so the hairs in the teat anlagen of 
the embryo are the same in number as the main milk-ducts, 
in the fully grown animal. 
Further, the distal, hollow part of the secondary outgrowth 
is lined by a double-layered epithelium, which is also 
characteristic of the tubules of the adult gland in repose. 
The solid cord of cells, on the other hand, is many cells in 
diameter, and it is characteristic of the main ducts that their 
walls are always more than two cells thick. Thus we may 
infer that the proximal part of the secondary outgrowths gives 
rise to the main milk-duct, while the terminal branches form 
