HISTOLOGY 



159 



thicker around the main and primary ducts and gradually becomes thinner 

 around the terminal ducts. 



Before puberty the gland consists of long ducts which have few side 

 branches. Shortly before puberty (between four to six weeks) more side 

 branches develop and the distal terminal branches end in enlarged end-bulbs 





Fig. 82. — Mammary gland of an eight weeks old mouse showing rapidly growing end- 

 bulbs. (X65.) 



lined by several layers of cuboidal epithelial cells which contain mitotic 

 figures (16, 35, 93) (Fig. 82). Increased mitotic activity and formation of 

 the end-bulbs of the distal ducts at the approach of each estrus has been 

 noted by several investigators. 



Fig. 83. — Developing mammary gland on the ii-th day of pregnancy. (X20C.) 



A gradual increase in the epithelial elements of the gland by cell division 

 is evident during the first part of pregnancy (Fig. 83). This increase reaches 

 its peak at about the iith-i2th day, and results in the formation of alveoli. 

 By the 14th to 15th day of pregnancy the alveolar system is well developed 

 and mitosis is infrequent. Further development consists of an increase in 

 size of the epithelial cells and an enlargement of the lumina of the ducts and 

 alveoli. Secretor}' activity is established gradually, starting first in the 

 alveoli proximal to the nipple and progressing distally. In the cytoplasm 



