214 J. A. MYERS 



but in individuals of the same species. In the rabbit, Rein 

 ('82) found the first vestige of a lumen in a very late fetus. At 

 five days after birth canalization is not entirely complete, but 

 at fifteen days the lumen extends to the tip of the nipple. It 

 does not open on the surface, however, owing to the presence of 

 cornified cells in the proximal end of the primary duct. Brouha 

 ('05) in the rabbit four days old found two of the milk-ducts with 

 lumina throughout, other ducts at the same age showing only 

 faint traces of lumina. At twenty-five days he found the 

 lumina completely formed for all of the ducts. In a kitten 

 twelve hours after birth Brouha found a part of the ducts pro- 

 vided with lumina. In Vespertilio murinus he found a trace 

 of a lumen in the milk-ducts of 20 mm. fetuses, while at birth 

 the lumina are quite well represented throughout the ducts. 

 De Sinety ('75) and Lustig ('16) found the lumina begin to 

 appear in human milk-ducts about the sixth or seventh fetal 

 month, but are not completely developed until birth or later. 



From the present work on rat fetuses and the foregoing ob- 

 servations of De Sinety ('75), Rein ('82), Brouha ('05), and 

 Lustig ('16) it may be concluded that the lumina of milk-ducts 

 usually begin to develop during the later fetal stages, but the 

 definitive lumen does not appear until birth or later. 



The earUest appearance of the lumen has been reported in 

 different parts of the milk-ducts. In the previously published 

 abstract of the results of the present paper (Myers, '17) it was 

 stated that the lumina make their earliest appearance in the 

 free ends of the milk-ducts. This statement agreed with the 

 findings of Rein ('82), Eggeling ('04), Raubitschek ('04), and 

 Lustig ('16). Further observations on a larger number of al- 

 bino rat fetuses, however, indicate that the lumina may appear 

 first in the excretory or external portions of the milk ducts, as 

 observed by Kolliker ('50) and Brouha ('05) in the glands of 

 the mouse, rabbit, cat and man. We must therefore conclude 

 that the first appearance of the lumina of the milk-ducts is 

 variable and may occur in various parts of the ducts. In the 

 rat, however, in the majority of cases the lumina show slightl}^ 

 further progress in development toward the free ends of the 

 ducts. 



