44 



THE EGGS OF MAMMALS 



other than the mouse has already been noted (Evans and 

 Swezy, 1931). According to Asami (1920) the rabbit ex- 

 hibits a constant rate of folUcular atresia before and after 



TABLE VI 



pregnancy. Pincus and Enzmann (19366) found that the 

 younger folUcles (types 1, 2 and 3 — Plate III) of the rabbit 

 show a much lower percentage of atresia than the larger 

 follicles. 



The atresia of mature ova can be prevented by pituitary 

 hormones. This is deduced from the phenomenon of super- 

 ovulation observed in animals receiving pituitary implants 

 (Smith and Engle, 1927; Smith, 1932). These authors 

 describe, for example, the presence of 49 ova in the tubes of 

 a mature mouse receiving anterior lobe implantations. An 

 adult mouse produces from six to twelve corpora lutea at 

 an ovulation, the absolute number varying with weight of 

 the mouse, the number of previous pregnancies, and certain 

 genetic factors (MacDowell and Lord, 1925; MacDowell, 

 Allen and MacDowell, 1929). In Smith and Engle's mice the 

 largest number of ova ever found in one tube of a normal 

 mouse was seven, and in an immature mouse showing super- 

 ovulation a maximum of 48 ova was observed in a single 

 tube. Thus the maximum number normally found in one 

 tube is 14.5 per cent of the maximum number super ovulated. 

 Furthermore, if we assume from MacDowell's data that 9 

 is roughly the number of ova normally ovulated this is 

 18 per cent of the 49 superovulated in the adult mouse. 

 These percentages agree with the estimations of per cent 



