adult herring live in water of a comparatively hig^ salinity; accordingly, the osmotic pressure of 

 the blood is also high. 



In herring maturing for the first time, the osmotic pressure of the blood evidently increases 

 as the gonads develop and the herring migrate westwards. It is assumed that in young, immature 

 herring living in water of a lower salinity the osmotic pressure of the blood is reduced. 



The Influence of Lig^t 



It has been proved by experiments, first in warm-blooded animals, later In fish, that the 

 cyclic development of the gonads is related to the ligjit conditions . 



In this connection Svetozarov and Shtralch (62) state that the influence of light in relation to 

 the sexual cycle in adult organisms has been investigated very thorou^y. In her paper on the 

 spawning of fish and its dependency on the surrounding medium, Polikarpova (58) stresses that fol- 

 lowing the work of Rouen (1929) stating the active influence of light on the development of the gonads 

 in vertebrates, a large number of papers has been published by various authors, confirming this 

 facti'. 



We shall linger a little on the examples given by Polikarpova. Hoover and Hubbert (1937) 

 stated that the length of the lig^t days had an Influence on the sexual products in trout . Under the 

 influence of light, they matured up to 4 months earlier than usual. The authors drew the conclusion 

 that it migtit be possible to direct the sexual cycle of the trout according to the influence of light . 

 Polikarpova admits that some fishes react on lig^t, but is of the opinion that the regulation of the 

 rhythm of propagation must largely be ascribed to the temperature of the water. She arrived at 

 this conclusion from experiments with small Callichtys fasciatus and guramy (Osphromenus tricho- 

 pterus) . The histological pictures of the ovaries were different in the control fish and in the experi- 

 mental fish. 



Other authors deny the influence of lig^t on the maturing of the sexual products (Gasterosteus 

 (Bennet), perch (Blssone) ). In common with Polikarpova, they consider the temperature as the most 

 importcint factor. 



Mathews (99), from experiments with Fundulus heteroclites concluded that during the period 

 of sexual repose light had no influence on the development of the sexual glands . 



Svetozarov and Shtralch infer that the lig^t only may act as a stimulus in cold-blooded ani- 

 mals under normal temperature conditions . Each change in the liglit regime in itself leads to a 

 disturbance in the development of the sexual glands. In cold-blooded animals, in which the degree 

 of self-regulation in this respect is less than in warm-blooded animals, the development of the 

 gonads is more dependent on the influence of the surrounding medium . Shtralch and Svetozarov (78) 

 observed the stimulating influence of the ligjit on frogs, shown in photomicrographs of the ovaries. 



Samochvatova (61) studied the influence of sunlight on the growth and reproduction of gam- 

 busia (Gambusia affinis holbrooki ) but came to a very uncertain conclusion. From the experiments, 

 she could not decide if the sunlight had any favorable influence on the fish, or if this influence work- 

 ed in an indirect way, related to a change in the rate of feeding. 



Smimov (63) compared the development of the sexual products in some species of Caspian 



\l Craig-Bennet (1931) (quoted by Svetozarov and Shtralch) investigated the Influence of llg^t on 

 the development of the sexual glands in the stickleback (Gasterosteus aculeatus ) and came to 

 the conclusion that the light was without any special influence . 



239 



