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DEVELOPMENT OF FISHES 



majority of sharks and rays are viviparous ; the eggs are 

 retained in the lowermost portion of the oviduct (uterus) 

 and the embryo establishes a "placental" circulation, the 

 vascular yolk sac becoming adherent to the walls of the 

 uterus. Other sharks deposit their eggs, and their mode 

 of oviposition has been observed. The egg (Fig. 189), 

 when slightly protruded from the cloaca, is rubbed against 

 brush-like objects, and when its terminal processes become 

 finally entangled, the egg is withdrawn. The processes of 

 the egg case which leave the body last, the longer ones, 

 are often greatly straightened out when the egg is depos- 

 ited ; subsequently their elastic character causes them 

 to curl tightly, and often to secure a firm attachment 

 to neighbouring objects. The eggs of oviparous skates 

 (Fig. 189 A) are said to be deposited on sand flats near 

 the mark of low water. Mr. Vinal N. Edwards of Wood's 

 Holl, Massachusetts, believes that they are implanted ver- 

 tically in the sand, and, from the occurrence of "beds" 

 of skate eggs, that the fishes are singularly local in their 

 places of spawning. Eggs of Elasmobranchs* are often 

 many months in hatching ; the young fish finally escapes 

 through a slit at the end of the egg case. 



Nothing is known definitely of the breeding habits of 

 Chimaeroids. The mode of copulation of the sexes is 

 doubtless similar to that of sharks. Their clasping organs 

 are highly specialized sperm ducts, and the hook-bearing 

 organs at the anterior margin of the ventral fin, and on 

 the forehead of the male, function in all probability in 

 retaining the female. The forehead spine could certainly 

 prove of such service if the position of the fishes during 

 mating was at all similar to that figured for Scyllium by 



* In the case of Scyllium the eggs are deposited about six days after they 

 have been fertilized ; they then hatch in from 200 to 275 days. 



