184 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finally entangled, the egg is withdrawn.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 
Chimzroids. 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 Scy//lium the eggs are deposited about six days after they 
have been fertilized ; they then hatch in from 200 to 275 days. 
