126 CYCLIC CHANGES IV 



rapid restitution of ova after repeated growth of the oviposi- 

 tor. We have therefore formed the opinion that the oviductin 

 produced stimulates the hypophysis into emitting a gonado- 

 trophic factor, which, in the gonad, causes development of 

 young oocytes, thus maintaining the appropriate number of 

 gametes (Fig. 83 B). 



We see, therefore, that in the course of the sexual cycle of 

 Rhodeus amavus, the ovary, as producer of gametes and as 

 hormonal gland, is linked with the hypophysis on the one 

 hand and the ovipositor on the other. In this, coordination 

 of the phenomena of estrus is characterized by a certain 

 rhythm in each of the 3 organs concerned. 



§ 2. THE SEXUAL CYCLE OF ZOARCES VIVIPARUS 



There are few fishes that possess such a clearly defined indi- 

 cator of the changes in their sexual-endocrine organization 

 as the ovipositor in Rhodeus. In most fishes oviposition as 

 such takes place in a very simple manner, and accordingly 

 we find only limited preparatory forms such as pre-ovulation 

 corpora lutea, and expansion of the oviduct. In viviparous 

 fishes, on the other hand, the care of the developing ovum 

 requires a number of accessory organs and numerous changes. 

 For this reason BRETSCHNEIDER and KristENSEN have 

 investigated the sexual cycle of Zoarces viviparus. 



Zoarces is one of the "ovary-breeders"; the ova are fer- 

 tilised in the ovary and there develop into young. Pregnancy 

 lasts about 4 months, and may take place in one animal from 

 April to July (pregnancy a), and in another from September 

 to January (pregnancy b) . Zoarces from the same locality 

 (North Sea, near Den Helder, North Holland) proved to be 

 subject to both pregnancies, although pregnancy occurred 

 with greater frequency in the autumn. 



It is improbable that a fish has two pregnancies in one 

 year. Since the animal material in our experiments appeared 

 to be mixed, the curves show both pregnancies within the 

 span of one year. We accordingly find, both in April and in 

 August and September, mature oocytes (Fig. 84), formed 



