STUDIES ON THE MAMMARY GLAND 3()9 



greater number of the collateral ducts of this gland pass so far 

 cephalad that onlj^ a comparatively small space exists between 

 them and the caudal collaterals of the second thoracic gland. 

 The caudal collaterals are few in number, yet quite long. 



The ducts of the abdominal glands (fig. 7), which are most 

 branched of all, send their branches in a latero-caudal direction. 

 Many collaterals are given off, more of which take a caudal than 

 a cephalic direction. In the case of the first inguinal gland, the 

 number of collateral branches taking the cephalic direction is 

 about equal to the number taking the caudal direction. The 

 ducts of the last inguinal glands send the majority of their 

 branches directly caudad; however, a few branches may be seen 

 passing cephalad toward the first inguinal gland. Terminal end- 

 buds are very prominent on all the glands at this stage. Also 

 the small lateral buds are numerous but they are largely confined 

 to the more distal ducts. 



Two weeks. Figure 2 represents all of the mammary glands of 

 a rat at the end of the second week of life. Figures 2 and S, 

 together with the description given in an earlier part of this 

 paper, render further description unnecessary. 



Three iveeks. The three weeks' specimen from which figure 9 

 was drawn shows less branching than the one-week stage, but 

 the ducts are greater in diameter. This specimen greatly em- 

 phasizes the marked variation in the development of the glands 

 in different individuals, since in this instance the glands of an 

 individual one week old show greater development than those of 

 another individual three weeks old. Also, as pointed out above, 

 the glands of one side may show more ad"\'anced stages of devel- 

 opment than corresponding glands of the opposite side. 



Four iveeks. At the end of four weeks the first pair of mam- 

 mary glands does not show a marked increase in development 

 over the two-weeks stage. One very noticeable difference is 

 that the proximal parts of the secondary ducts bear a large num- 

 ber of lateral buds similar to those appearing on the distal parts 

 of the same segment of the earlier stages. Such processes are 

 much less numerous on the corresponding parts of the second and 

 third thoracic glands. The second glands in some of the speci- 



THf: AMERICAN JOURNAL OF ANATOMY, VOL. II), NO. 3 



