PLATE 1 



Figs. 1-6 represent one-half to two-thirds of the first right inguinal mammary 

 gland removed from female rats after various treatments as listed below. Fixed 

 in lo per cent formol, stained with alum-carmine, cleared in methyl salicylate, 

 photogiaphed at x 1.25. 



Hypophysectomy and oophorectomy were performed on the same day when 

 the rats \v'ere 60-70 days of age; treatment begun immediately and continued for 

 10 days with necropsy on day 11. 



Fig. 1. From a hypophysectomized rat injected with FSH, glucose, and 

 thyroxin. This is the state of mammary development referred to in the text and 

 table as D, signifying that little or no parenchyma beyond bare ducts is present. 

 It is representative of the slow regression that occurs following hypophysectomy 

 in the rat used, whether uninjected or injected with certain levels of estrin, 

 progestin, or FSH. 



Fig. 2. From a hvpophysectoniized-oophorectomized rat injected with crude 

 lactogenic hormone, FSH, thyroxin, and glucose. Referred to as DAB = ducts, 

 and a few scattered alveoli which are mostly in the so-called "bud" form and not 

 aggregated into lobules. The development shown here represents the upper 

 limit of what may be found in normal 60-70-day-old rats. 



Fig. 3. From a hypophysectomized rat injected with purified lactogenic hor- 

 mone, FSH, thyroxin, and glucose. Referred to as LA-I- = minimal lobulo- 

 alveolar growth which may also be induced with pure lactogenic hormone and 

 estrone. 



Fig. 4. From a hypophysectomized-oophorectomized rat injected with estrone, 

 progesterone, crude lactogenic hormone, thyroxin, and glucose. Referred to as 

 LA-H-, signifying an intermediate degree of lobulation. 



Fig. 5. From a normal rat injected with purified lactogenic hormone. Referred 

 to as LA4-H-. This gland and that in figure 6 represent the variation seen in the 

 pregnant rat from day 16 to term. The degree of development shown here has 

 been attained in hypophysectomized rats injected with crude lactogenic hor- 

 mone plus either FSH or estrone. 



Fig. 6. From a normal rat injected with crude lactogenic hormone. Referred 

 to as LA I I I I = complete lobulo-alveolar growth. Enlargement of individual 

 alveoli suggests that the early secretory phase had begun in this gland. 



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