STUDIES ON THE MAMMARY GLAND 423 
tory alveoli of the gland with their simple epithelial lining do 
not make their appearance until the last few days of pregnancy 
or until some time after ovulation when this has not been fol- 
lowed by fertilization.” The work of these investigators, the 
present work, and my previous work (Myers, 716) prove that in 
certain animals true alveoli do not usually appear before puberty. 
It has been proved quite conclusively that true secretory 
alveoli do appear after the first ovulation. In fact, Ancel and 
Bouin (711) and Schil (12) showed that in the rabbit the mam- 
Fig. 11 Internal view of a wax model reconstructed from two terminal duct 
and end-buds of the right first thoracic gland of a female albino rat of seven weeks. 
e.b., end-bud; ep.w., epithelial wall; 7., lumen. X 100. 
3 
mary gland develops after each ovulation not followed by preg- 
nancy to the same extent as it develops during the first half of 
pregnancy. O’Donoghue (712) and Hartman (’18) reported 
somewhat similar changes in the mammary glands of Dasyurus 
and the opossum. Most of the investigators believe this develop- 
ment is due to the corpus luteum. The enlarged terminations of 
the ducts which resemble alveoli in some ten-week rats were 
perhaps developed during the first period of heat or some time 
after the first ovulation. 
