STUDIES ON THE MAMMARY GLAND 213 



present at twenty days. Very rapid growth takes place be- 

 tween the twenty day stage and the newborn, as my reconstruc- 

 tions and cleared preparations (Myers '16) show that the ducts 

 are much elongated and several new divisions have occurred in 

 the latter. 



The first few divisions of the milk-ducts in the twenty day 

 fetus (fig. 12) follow the true dichotomous method of branching. 

 The divisions farther away from the primary ducts, however, 

 do not come ofT so regularly, yet they present a very irregular 

 form of dichotomy. The same condition obtains in the newborn 

 and later postnatal stages (Myers '16). Langer'('51), Kolliker 

 ('79), and Lustig ('16) found that for the most part the milk- 

 ducts of human fetuses branch dichtomously. Kolliker ('79) 

 states that the human manmiary ducts branch two to eight 

 times by the true dichotomous method after which the branch- 

 ing is somewhat irregular. The method of branching of the 

 milk-ducts of the albino rat, therefore, appears to be similar 

 to that of the human. 



The terminal end of each milk-duct in all stages of the rat 

 fetus studied presents an enlargement. Langer ('51) noticed 

 such enlargements at the terminal ends of developing milk- 

 ducts in the human, and they have since been reported by num- 

 erous investigators. Formerly such terminal swellings were 

 believed to be true acini. The present work, however, as well 

 as my previous study (Myers, '16), confirms the view that they 

 are not true acini, but are merely growing end-buds. 



The first indication of a lumen in the ducts was observed in 

 a rat fetus of eighteen days and nine hours. The lumina ap- 

 pear, however, in only a part of the ducts observed at this stage, 

 while at twenty days and six hours the majority of the ducts 

 show lumina in an early stage of development. At birth the 

 lumina extend from the intra-epidermal portion of the primary 

 ducts to within 20 or 30 micra of the free extremities of the ter- 

 minal ducts. Such lumina, however, have not yet reached their 

 definitive state. 



The time of development of the lumen in the mammary ducts 

 is subject to considerable variation, not only in different species 



