A treatise on zoology . e oesophagus and in front of the liver. The Miillerian duct (oviduct) is developed in both sexes, re-maining as a vestige in the adult male. It is derived from thepronephros (Balfour [27], Eabl [337]). The rudimentary pronephricfunnels combine to one opening, shifting backwards to a positionbehind the pericardial septum (p. 90). The duct is .split off fromthe archinephric duct, which thus becomes divided into an oviductor Miillerian duct, and a mesonephric or Wolffian duct. As a rule,the egg is large, heavily laden with yolk ; fertilisation is internal,and cleavage mero
A treatise on zoology . e oesophagus and in front of the liver. The Miillerian duct (oviduct) is developed in both sexes, re-maining as a vestige in the adult male. It is derived from thepronephros (Balfour [27], Eabl [337]). The rudimentary pronephricfunnels combine to one opening, shifting backwards to a positionbehind the pericardial septum (p. 90). The duct is .split off fromthe archinephric duct, which thus becomes divided into an oviductor Miillerian duct, and a mesonephric or Wolffian duct. As a rule,the egg is large, heavily laden with yolk ; fertilisation is internal,and cleavage meroblastic. A large narrow-stalked yolk-sac remainsfor a considerable time protruding from the ventral surface of theembryo, often even after birth. A lobe of the yolk-sac may alsolie in the abdominal coeloni. The vitello-intestinal duct arises fromthe anterior end of the intestine. A characteristic horny case, secreted l>y a special glandularregion of the oviduct (Fig. 91), and containing an albuminous DE VELOPMENT 133. Fio. and of tislies. (From Dean.) A, Iltkllogtoma, egg-case; B, upper pale oftVie same showing hooks and micropyle (after Ayers); C, Myxim (after Steenstriip); D, aprocess of the same; B, Petromyzon ; P, ScyUlum (after ); G, rMju ; H,Heterodmtus (after Gunther); 1, same enlai-ge<l (arti <;ui(il). 134 ELASMOBRANCHII fluid, surrounds the egg when it is laid. In Laemargiis, however,fertilisation appears to be external, and the eggs are comparativelysmall. This is probably a secondary modification. Many Selachians are viviparous. In these, the egg-case becomesreduced to a thin membrane {Mustelus) or disappears altogether{Torpedo), and the wall of the oviduct develops bunches ofsecreting Ailli or cotyledons (Dmneril [124]). While the
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