STUDIES ON THE MAMMARY GLAND 205 



over the mammary glands a thickened appearance. Later the 

 cornified cells were cast out and thus the funnel corresponding 

 to the mammary pit was deepened. The anlage of the nipple 

 in the present stage seems to have pushed from the bottom of the 

 mammary pit toward the surface leaving a surrounding furrow or 

 sulcus (figs. 3 and 9, s.). The superficial part of the epidermis 

 over the nipple anlage now appears no thicker than that in 

 adjacent regions. 



The anlage of the epithelial hood has grown more deeply 

 into the corium now encroaching upon the tela subcutanea. 

 Cornified epithelial cells occupy the space between the inner and 

 outer surfaces of the hood. 



\'\Tien the stratum germinativum of the epidermis is traced 

 toward the region of the mammary gland it is seem to pass 

 deeply and form the outer surface of the epithehal hood. It then 

 covers the free edge of the hood and turning back forms the 

 inner surface of the hood. Next it covers the deep surface of 

 the nipple anlage. Throughout its extent in the mammary 

 region the cells of the stratum germinativum rest on a base- 

 ment membrane (fig. 3). 



The corium mthin the epithelial hood is composed of con- 

 nective tissue cells the processes of which take a deep blue stain 

 when treated with Mallory's connective tissue stain. Small 

 blood vessels and nerves are also included. From the surface 

 of the nipple anlage the primary duct is seen coursing through 

 the corium in the center of the hood on its way to the subcutaneous 

 tela w^here it turns at right angles after which it lies parallel 

 with the surface of the integument. Soon after reaching the 

 subcutaneous tela and turning at right angles the duct divides 

 into secondary ducts each of which in turn divides into tertiary 

 ducts. Quaternary ducts are beginning to arise from the ter- 

 tiary ducts (fig. 12). The terminal ducts present small knob- 

 like enlargements or end-buds. Not every gland observed at 

 this stage presents all of the above mentioned branches. For 

 example in the second inguinal gland of one specimen the pri- 

 mary duct has divided into two secondary ducts which remain 

 undivided. 



