STUDIES ON THE MAMMARY GLAND 375 



definite epithelial hood is formed, as shown in figures 3 to 6. 

 Through this hood, which is filled with loose connective tissue, 

 the primary duct passes on its way toward the surface. 



Very little change takes place in the appearance of the nipple 

 during the first week of postnatal life. From the surface the 

 nipple areas appear slightly more elevated than at birth. Fig- 

 ure 15, drawn from a section cut very obhquely, shows that the 

 thickened area of epithelium has become more extensive, also 

 that the epithelial projections (ingrowth) are slightly longer. 



During the second week the nipple develops very rapidly. It 

 is no longer necessary to speak of it as the nipple area, since from 

 its size and shape it now resembles a true nipple. Figure 16 

 shows the nipple at the age of two weeks to be a very prominent 

 structure with the epithelial projections (ingrowth) correspond- 

 ingly well developed. At this stage the primary duct actually- 

 pierces the surface. As previously stated, the nipples are very 

 conspicuous externally toward the end of the second week of 

 life, and from the tenth to the fourteenth days it was found 

 easier to observe the glands externally than at any other time 

 during the virgin state. During the third week, the nipples be- 

 come hidden by the development of the hair coat. 



Figure 16 also shows that at the age of two weeks a sulcus 

 is beginning to appear between the nipple and the skin of the 

 surrounding region. This sulcus is immediately superficial to 

 the epithelial ingrowth mentioned above. Beyond the two- 

 weeks stage (up to ten weeks) no changes have been observed to 

 take place in the nipple, except that it slowly increases in size. 

 This is shown in a nine-weeks stage (fig. 17) in which the struc- 

 ture is similar to that in figure 16, the chief difference being one 

 of size. 



SUMMARY 



The results of the present investigation of the growth and 

 gross relations of the ducts and the nipples of the albino rat from 

 birth to the tenth week may be summarized as follows: 



1. Observations made on 100 rats show the number of glands 

 varies between 10 and 13, the normal number being 12 (6 pairs). 



