336 FISHES 



has not been actually observed in these fishes it is known to take 

 place several months before the eggs are fertilised, namely in June 

 or early July. After this time the ovary of the adult female 

 contains a quantity of milt, and the eggs mature and are fer- 

 tilised between the beginning of November and the end of Jan- 

 uary, those of the larger and older individuals maturing earlier. 

 The mature eggs are of extremely small size in comparison 

 with those of other fishes, being only *2 millimetre, or xl-th 

 of an inch in diameter. This reduction in size is due, as in the 

 case of the eggs of mammals, to the almost entire absence of 

 yolk, the embryo being supplied with nourishment from the 

 ovary after development has commenced. The egg is fertil- 

 ised while still within the follicle, but immediately afterwards 

 it becomes free and lies in the cavity of the ovary. The dura- 

 tion of gestation is about five months and the number of young 

 in a single female varies in Cymatogaster aggregatus from three 

 to twenty according to the size of the mother ; in other species 

 there may be from twenty to thirty young in one female. The 

 two ovaries are united into one sac, and from the dorsal wall of 

 the sac project several longitudinal folds of membrane, which 

 are richly furnished with blood-vessels and extend among 

 the embryos, supplying them with oxygen and material for 

 their nourishment. At first absorption is effected by the 

 general surface of the embryo, then when the intestine is 

 developed the ovarian secretion is taken into it and digested. 

 Long before the mouth is developed the first pair of gill-slits is 

 opened and the food current enters through these in conse- 

 quence of the movement of minute processes, technically called 

 cilia, which are developed on the walls of the gullet. At a later 

 stage when the mouth is opened the hind-gut becomes enor- 

 mously enlarged, and on its inner surface are developed very long 

 absorptive processes or villi which are special adaptations for 

 the absorption of the ovarian secretion. There is also a special 

 adaptation in the later stages for the respiration of the embryo : 

 when the fins are developed their membranes are extended 

 beyond the extremities of the fin-rays and are supplied with a 

 close network of blood-vessels by which oxygen is absorbed 

 from the maternal blood and carbon dioxide given out. The 

 development of these respiratory membranes corresponds to 

 that of the scales on the sides of the body ; when the scales are 



