STUDIES ON THE MAMMARY GLAND 219 



2 About the seventeenth or eighteenth day the deep portion 

 of each anlage elongates into a long solid cord of epithelium — 

 the anlage of the primary duct. At this time each anlage is only 

 about 0.05 mm. in length. The free end of each primary duct is 

 directed toward the position which the future system of ducts 

 will occupy. At eighteen or nineteen days each of the primary 

 ducts present two secondary ducts. About the twentieth day 

 tertiary and quaternary ducts are present. The first few divi- 

 sions are usually dichotomous, while the more distal ones become 

 somewhat irregular. Growing end-buds are present on the free 

 ends of the terminal ducts. 



3. Between the eighteenth and nineteenth days an epithelial 

 projection grows in from the stratum germinativum around each 

 gland area. Each projection extends entirely around the pri- 

 mary ducts thus forming the epithelial hood. 



4. The mammary pit first appears on the surface as a slight 

 depression over each developing gland. It is apparently formed 

 by the processes of cornification and desquamation of the thick- 

 ened epithelium. The pit is well developed at nineteen or 

 twenty days. 



5. The nipple anlage was first observed in twenty day and six 

 hour fetuses. At this stage it is a small papilla-like eminence 

 lying at the bottom of the mammary pit. The nipple reaches 

 only a rudimentary stage of development in the prenatal stages 

 of the albino rat. 



6. The lumina of the ducts were first observed in eighteen day 

 and nine hour fetuses. They were not confined to any definite 

 part of the system of ducts, but usually appeared shghtly better 

 developed toward the free ends of the ducts. The lumina do 

 not reach their definitive stage in the fetal state. In the fetusee 

 examined, the lumina are apparently formed by a rearrangement 

 of the cells, thus producing numerous lacunae which later flow 

 together to form the incomplete lumina of the latest stage studied. 

 No traces of cell degeneration were observed. 



7. In the earliest stages studied the mesenchymal cells are 

 condensed around the mammary gland anlage. Later these 

 cells elongate and develop long fibrous processes. At twenty 



