THE MAMMARY GLAND. 447 



most definite and unmistakable form of vacuolation is the sig- 

 net-ring type." This process is, according to him, a true one 

 of endogenous cell formation, resulting in this instance in the 

 formation of milk. Moreover, large, granular, nucleated cells, 

 filled with a bright yellow or golden pigment, "found both 

 within the alveoli and in the interfibrillar spaces without them" 



FIG. 194. Vacnolation of alveolar epithelium. From the udder of a ewe shortly after the end of lac- 

 tation. The cells in situ are vacuolated cells, with the usual thin and, for the most part, uncolored hoop 

 or ring of the vacuole, and the deeply stained peripheral mass. Creighton. 



characterize the last stage of involution, "and the pigment 

 that belongs to them is to be found strewn over the lobules 

 that have reached the resting state." Finally, Creighton as- 

 serts that " the various forms of cells that characterize the 

 various stages of involution must have resulted from a trans- 

 formation de novo of the renewed epithelium, and not from 

 successive changes upon the same cell." Each epithelial cell, 

 therefore, that is used up in the formation of milk, has been at 

 one time a perfect polyhedral corpuscle or fully equipped cell, 

 and "has rapidly undergone the cycle of changes whereby 

 its whole substance has been converted into milk." 



A distinguishing feature of one stage of evolution which 



