24 THE EGGS OF MAMMALS 



decline of ovogenesis with age in adult females (see Fig- 

 ure 6) . 



The experiments of Engle (1928) demonstrate adequately 

 that pituitary secretions are responsible for the later stages 

 of maturation. He injected anterior lobe tissue into normal 

 and semi-spayed rats and found that the per cent of hyper- 

 trophy due to pituitary stimulation was approximately equal 

 in the two groups of animals. We have already noted that 

 in compensatory hypertrophy the increased ovarian weight 

 is due to the doubling of large follicle and corpus luteum 

 number, the number of primary follicles being the same in 

 a single ovary whether the second ovary is present or 

 not. 



Swezy (19336) also determined the effect of various pitu- 

 itary hormone preparations upon ovogenesis in adult and 

 immature rats. Her data are collected and summarized in 

 Table II. 



Immediate verification of the conclusions deduced from 

 Table I is found in the data derived from the injection of 

 rat hypophyses into adult and immature rats (columns [11, 

 [10] and [11]). Rat hypophyses are notably rich in gonad 

 stimulating hormones (Smith and Engle, 1927), and their 

 administration results in a decrease in the rate of ovogenesis, 

 and an increase in total ovarian tissue. The data on the 

 immature rats are particularly striking, for a few days of 

 pituitary administration results in a halving of the total 

 number of ova. Arai (1920a) found that the average total 

 number of ova in prepubertal rats was about 10,000 and 

 approximately 6000 in post-pubertal animals. 



Beef hypophyses, on the other hand, are relatively poor 

 in maturity hormone and rich in growth hormone. Evans 

 and Simpson (1928) have demonstrated an antagonism be- 

 tween the growth and gonad-stimulating hormones of the 

 anterior pituitary. The increase in follicle number following 

 beef hypophysis administration (column 2) might then be 

 interpreted as a neutralization of the intrinsic maturity 

 hormone effect by the growth hormone of the beef pituitary. 



