IV. THE CYCLIC CHANGES IN THE SEXUAL- 

 ENDOCRINE ORGANIZATION OF NON- 

 MAMMALIAN VERTEBRATES 



The aim of our investigations in the sexual-endocrine field 

 was to elucidate the sexual cycle. We first studied the sexual 

 cycle of the female bittevling (Bretschneider, Duyvene 

 DE Wit, Goedewaagen, Van Iersel, and Meltzer), in 

 which growth of the ovipositor clearly indicates the internal 

 condition of the sexual-endocrine system. Following this, 

 we studied the sexual cycle of other non-mammalian ver- 

 tebrates, e.g., that of two viviparous fishes, Lebistes reticularis 

 (Bretschneider, Jaski) and Zoarces viviparus (Kristen- 

 SEN, Bretschneider), and that of Bufo bufo (Bret- 

 schneider). Our investigations into the sexual cycle of 

 Tropidonotus natrix and Seps chalcides (BRETSCHNEIDER) , 

 and Serinus canatia (Lever) , are not yet completed. 



§ I. THE SEXUAL CYCLE OF RHODEUS AMARUS $ 



By a sexual cycle we mean the succession of changes occurring 

 in the sexual apparatus, in estrus and in anestrus. Since 

 Rhodeus belongs to the monestrous type, the cycle extends 

 over an entire year. We have taken the beginning of anestrus 

 as the starting point of our description of the cycle, since, at 

 this time, sexual activity is at its lowest level. 



Estrus lasts about 2 months (May and June, or, if Spring 

 is early, estrus may begin about the middle of April) ; and, 

 as described above on p. 70, it is rhythmical. Anestrus lasts 

 8 months (July to February); and pro-estrus, or the pre- 

 paratory phase, two months (March and April). We have 

 examined the following stages in the complicated cyclic 

 process: (a) ovogenesis; (b) corpus luteum formation; (c) 

 the pituitary regulation of both, and (d) growth of the 



