GROWTH-CHANGES IN MAMMARY APPARATUS OF DASYURUS. 205 
there is also an enlargement of the teat, which has grown 
from about 2-3 mm. up to 8 mm. in number 24 and 1 cm. 
in number 25. 
In No. 23 the gland is in a very interesting condition, 
for although it is, on the whole, in a more advanced state 
than in any of the preceding stages, the alveoli are very much 
smaller, but still in a fully active secretory condition. On 
the other hand, the main ducts and their proximal branches 
are enormously distended, and quite full of milk (fig. p 9). 
This is explained by the fact that the milk is periodically 
forced into the throat of the embryo by the contractions of 
the cremaster muscle, and that this particular animal was 
killed while this feeding process was taking place. 
The Nature of Milk Secretion. 
Two opposing views have been put forward with regard to 
the nature of milk secretion : it is maintained, on the one 
hand, that its production is necrobiotic, and, on the other 
hand, that it is purely a normal secretory process. 
The first view was propounded by Virchow ( 48 ), who held 
that the activity of the mammary glands was similar to that 
of the sebaceous glands, and consisted of a physiological fatty 
degeneration, in the course of which the epithelial cells fell 
into the lumen and by their disintegration set free the milk- 
globules. Langer ( 32 ), however, came to the conclusion that 
the death of the epithelial cells played no part in milk forma- 
tion, which was, on the contrary, an entirely secretory occur- 
rence. This point of view was endorsed by Schmidt ( 43 ), 
who thought, however, that after some time necrobiosis set 
in. An intermediate position between these two extremes 
was adopted by Heidenhain (22) in criticism of a theory 
propounded by Rauber ( 38 ), that the leucocytes invaded the 
alveoli and there underwent degeneration to form milk, and 
the former maintained that the activity was partly necrobiotic 
and partly secretory. This writer also pointed out that if the 
activity was purely necrobiotic it would follow that, in order 
