STUDIES ON THE MAMMARY GLAND 419 
alveoli present in these stages. Schil (712) states that from birth 
until shortly before the puberty period the mammary gland of 
the rabbit undergoes very little or no growth, that the slight 
elongation of the ducts is due to a stretching produced by the 
developing stroma. He states that there are two facts which 
characterize this period: first, the gland maintains the develop- 
ment already reached at birth; second, some.of the cells produce 
a very slight secretion. These facts, he believes, prove that the 
gland does not receive any impulse of growth or secretion. How- 
ever, Schil thinks that during this entire period the glands are 
under the influence of the ovaries, for after double ovariotomies 
in young animals the free parts of the ducts not only fail to pro- 
duce a slight secretion, but disappear so that the gland is re- 
duced to some short ducts centering around the nipple. On 
the other hand, he cites the experiments of Knauer and Foges 
which show that after simple hysterectomies without ablation 
of the ovaries the gland remains as in normal or control animals. 
Schil concludes, therefore, that the presence of the ovary is 
necessary to maintain the mammary gland in the stage it has 
reached at the time of birth. 
My previous work (Myers, ’16) shows beyond doubt that the 
ducts do grow and proliferate a great deal after birth and before 
the period of puberty is reached. According to the castration 
experiments of Steinach (’12) and others, it seems probable that 
this gradual growth in the milk-ducts of the albino rat from 
birth to a short time before puberty is under the influence of the 
ovaries. Further castration experiments are being attempted 
to confirm this point. It has been pointed out, however, (Myers, 
’17 b) that the milk ducts of the male albino rat undergo a similar 
but less extensive growth and proliferation from the time of 
birth to puberty. If, as Schil believes, the presence of the ova- 
ries is necessary even to maintain the gland of the rabbit in its 
infantile state until puberty, we are at present unable to locate 
the factor which stimulates the growth of the glands of the male 
during this period. 
The rapid growth and proliferation of the milk-ducts at the 
time of puberty has been mentioned by a number of investigators, 
