STUDIES ON THE MAMMARY GLAND 211 



('primitive Zitzen'), a misleading term since, as he observed, 

 they are merely transient structures. 



Similar hillocks have been observed in human embryos by 

 Langer ('51), Rein ('82), Brouha ('05), Lustig ('16), and others. 

 They have been described by Rein ('82), Schultze ('92 and '93), 

 Bonnet ('92) and Brouha ('05) in the following species: pig, 

 sheep, dog, fox, cat, rabbit, squirrel, rat, mouse, and mole. 

 The name mammary hillocks ('Milchhiigeln') was applied to them 

 by Bonnet ('92). 



The depression or fossa (mammary pit) which forms over each 

 developing gland resembles the pocket which contains the 

 nipple in some marsupials and which Owen ('68), Gegenbauer 

 ('73) and others believed to exist in Monotremes. Bresslau in 

 1908 proved the non-existence of such a pouch in echidna and 

 ornithorhynchus. In an earlier work, however, Bresslau ('02) 

 observed that a definite pocket ('Zitzentasche') developed in 

 some marsupials in the region of the future nipple. Bresslau's 

 findings in marsupials confirmed the work of Klaatsch ('84) 

 and others who showed that in marsupials a fairly deep pocket 

 is developed in the region of each mammary gland; and at the 

 bottom of each pocket a small papilla-like eminence occurs 

 which is believed to be the first appearance of a nipple in mam- 

 mals. During the resting phases of the glands the nipples 

 remain in the pocket, but they actually protrude from the pocket 

 and may be drawn out to a considerable extent while the glands 

 are active. 



The ontogeny of the mammary gland nipple of the albino 

 rat apparently repeats in most respects the above described 

 conditions in the lower forms of mammals. In the rat we have 

 seen the surface over the future nipple region excavated (chiefly 

 by the processes of cornification and desquamation) so as to 

 form a definite pocket (figs. 8 and 9), the mammary pit. At 

 the bottom of this pit is seen in sections the proximal end of 

 the primary duct. Later a papilla-like elevation (the nipple 

 anlage) appears at the bottom of the pit. At this time the 

 nipple is so small that it occupies only a part of the pocket. 

 At birth the nipple has enlarged so as to fill the pit, with the 



