MODES OF REPRODUCTION 339 



while Sebastes has eggs resembling those of open-sea fishes 

 which are small, numerous, with little yolk, and usually buoyant. 

 The egg of Zoarces is 2*5 millimetres in diameter. The num- 

 ber of eggs in a single female varies with the size of the mother 

 but is always much smaller than in Sebastes and its allies ; 

 females of seven or eight inches in length produce from twenty 

 to forty young at a birth, those of eight to ten inches from fifty 

 to one hundred and fifty, while larger specimens have been found 

 to contain three hundred young or even more. The period of 

 development within the vitelline membrane was found by Van 

 Bambeke in Belgium to be about three weeks : he found eggs 

 in the ovary in very early stages of development on the nth 

 September, and the first hatched embryos on 27th Septem- 

 ber. It is of course impossible to observe in viviparous fishes 

 the course of development on the same eggs, since a single ob- 

 servation involves the death of the mother and consequently of 

 the eggs also. The fact however that eggs were only beginning 

 to develop in September indicates either that copulation takes 

 place then as well as in spring or that fertilisation does not 

 occur until some time after copulation. The embryo, accord- 

 ing to Bambeke, remains in the ovary for a considerable time 

 after hatching, the total duration of gestation being about four 

 months. The young at birth are 1^ in. to 2 in. in length and 

 are fully developed. There can be little doubt that the embryo 

 derives nourishment from the ovary in addition to that supplied 

 by the yolk, but no special adaptations for nutrition or respira- 

 tion have been described. 



The numerous species of the carp family produce adhesive 

 eggs which are attached to aquatic weeds, but they take no 

 care of them. One species of this family has very curious breed- 

 ing habits. It is a little fish called in Germany the bitterling 

 from its bitter taste (Rhodeus amarus). It is only two to three 

 inches long when full grown, and has large smooth scales. The 

 female is provided with a long tube extending from the genital 

 opening, which serves as an ovipositor, and by means of this the 

 fish introduces the eggs into the cavity within the shell of the 

 fresh- water mussels (Anodon and Unto). In this cavity, where 

 they are well protected from enemies, the ova undergo their 

 development. The respiratory current of water of the mussel 

 provides oxygen for the ova. The eggs are fertilised by the 



