374 J. A. MYERS 



appear greater in diameter than that of other parts of the ducts 

 but the walls of the bud are usually considerably thickened. In 

 some places the lumina of the ducts at birth are partially filled 

 with substance which resembles colostrum. 



At the end of the first week, the lumen is considerably larger 

 and extends farther into the intra-epidermal segment. Con- 

 siderable quantities of the substance mentioned above are pre- 

 sent in the lumina of all segments of the ducts. 



Figure 16 shows that the lumen opens on the surface of the 

 nipple at two weeks. At this stage it is larger and more oval 

 throughout the primary duct than in the previously described 

 stages. 



In other stages up to the tenth week no marked differences 

 were observed except that the lumina gradually become larger 

 as the ducts increase in size (fig. 17). No true alveoli were ob- 

 served, yet the end buds in the later stages are changed some- 

 what in appearance so as to approach, or at least suggest, begin- 

 ning alveoli. In all the stages more or less of the substance 

 mentioned in the earlier stages was observed. This agrees with 

 findings of Berka ('11), who describes particles of fat in the 

 human ducts even in the later virgin stages. He thinks the 

 source of this fat is the colostrum secretion of the newborn and 

 that it lies in the ducts for years. 



4- Growth of the nipple 



Owing perhaps to the fact that the rat is born in a somewhat 

 immature state, the nipples show only slight development at the 

 time of birth. In the newborn the nipple can be recognized by 

 the naked eye only with difficidty, the nipple areas appearing 

 slightly lighter than the surrounding tissue. A section through 

 the nipple area at this stage reveals the fact that it is only very 

 slightly elevated above the surface of the skin (fig. 14). The 

 epithelium of this area is considerably thickened and the figure 

 reveals an epithelial ingrowth on each side of the thickened por- 

 tion. Reconstructions show that these epithelial projections 

 are continuous around the nipple area and that through them a 



