STUDIES ON THE MAMMARY GLAND 215 



The manner in which the lumen is formed has hkewise been 

 a subject of considerable controversy. It will be recalled that 

 in the rat fetus of about the eighteenth or nineteenth day small 

 irregular intercellular cavities or lacunae appear in the epithe- 

 hum of the milk-ducts. The lacunae are chiefly confined to 

 the center of the developing ducts, but may occur peripherally. 

 The cells and their nuclei in the region of the lac;unae show no 

 signs of degeneration. A little later the lacunae flow together, 

 thus forming a lumen which is in a very incomplete stage of 

 development at this age. The lumina are better developed at 

 birth (Myers, '16), but are still incomplete. De Sinety ('75), 

 Rein ('82), and Keiffer ('02) have described the formation of 

 the lumina in human as a process of degeneration. They state 

 that the central cells of the solid epithelial duct anlage degene- 

 rate, the debris being found in the developing lumina. My 

 fetal stages show no such condition, but agree rather with the 

 findings of Benda ('94) and Brouha ('05), who described the 

 formation of the lumen in the mouse, rabbit, cat and man as 

 a process of cell-rearrangement, rather than cell-degeneration. 



The epithelial hood 



The anlage of an epithelial ingrowth or hood was first observed 

 in one of the abdominal glands of an eighteen day and nine hour 

 rat fetus. Such anlages are present in most of the glands in 

 ninteen day fetuses. These anlages were seen to bud off 

 from the deeper epithelial surface funnel-shaped mammary pit. 

 About the twentieth day the ingrowth forms a real hood around 

 the proximal end of each primary duct. When examined 

 microscopically, the part of the hood attached to the walls of 

 the mammary pit is seen to be filled with a thin layer of corni- 

 fied cells which is continuous with the mass occupying a part of 

 the mammary pit. No cavity is yet present in the hood, al- 

 though its attachment corresponds to the region of the sulcus 

 between the nipple anlage and the wall of the manamary pit. 



The epithelial hood has been observed by several investi- 

 gators (Gegenbauer, '7(3, Rein, '82, Klaatsch, '84, in rodents and 



