212 J. A. MYERS 



exception of a shallow sulcus which still surrounds the nipple. 

 The nipple in the newborn rat thus produces a slight eminence 

 on the surface of the skin. In an earlier paper (Myers, '16) 

 my low power drawings do not show the sulcus around the nipple 

 in rats at birth and one week of age. This is due to the fact 

 that over the sulcus the epidermis is slightly thickened, and also 

 because the sulcus contains some cornified cells. Nevertheless 

 under high power the sulcus is still very evident in these postna- 

 tal stages. 



The mammary pit which develops before the appearance of 

 the nipple is apparently homologous with the nipple pocket 

 which Gegenbauer (73 and. 76), Rein ('82), Klaatsch ('84), 

 Bresslau ('02), and many others observed especially in mar- 

 supials. Bresslau ('02) believed that the mammary pit is 

 homologous with the marsupial pouch. Later, however (Bress- 

 lau '10), he regarded it as a homologue of the nipple pocket 

 of marsupials. 



The milk ducts 



In the rat fetuses the anlage of the milk duct was first ob- 

 served about the seventeenth or eighteenth day. At this time 

 the deep part of each epithelial mammary gland anlage apparently 

 elongates or sends out a single bud-like process which is the 

 primary duct anlage. This stage may be said to correspond to 

 Rein's ('82) period of bud formation ('Kjiospenbildung') in 

 rabbits. It differs, however, from the findings of Langer ('51), 

 Huss ('71), Kolliker ('79), Rein ('82), Profe ('98), Hamburger 

 ('00), Brouh'a ('05), Lustig ('16), and many others in that they 

 observed a variable number of buds (primary duct anlages) in 

 man and other animal species including the horse, pig, cat and 

 rabbit. On the other hand it agrees with the observations of 

 De Sinety ('77), Gegenbauer ('76), Klaatsch ('84), and Brouha 

 ('05) who reported the existence of a single primary duct in 

 rodents and insectivorous mammals. 



Between the eighteenth and nineteenth days each primary 

 duct in the rat fetus presents two secondary ducts. The second- 

 ary ducts later present tertiary ducts. Quaternary ducts are 



