i6o 



BIOLOGY OF THE LABORATORY MOUSE 



small droplets of secretion appear which gradually fuse into large drops. 

 The nuclei are pushed toward the base away from the lumina. At 17 to 

 19 days of pregnancy secretory activity is generally well established (Fig. 



Fig. 84. — Mammary gland showing secretory activity near the end of pregnancy. 



(X200.) 



84). (Parturition at 20 days.) Parallel with the glandular development 

 there is an intensive development of blood vessels with which developing 

 ducts and alveoli come in intimate contact. As the glandular parenchyma 



Fig. 85. — Mammary gland on the 7-th day of lactation. (X 100.) 



occupies more and more space, the adipose cells of the fat pads rapidly 

 disappear. 



During lactation the ducts and alveoli are dilated and contain milk 

 (Fig. 85). The original lobes and lobules of the fat pad supply the frame- 

 work of the lactating gland, and adipose cells serve only to fill in the space 



