REPRODUCTIVE ORGANS 407 



versa? To this question no decisive answer can at present' be 

 given. 



The terminal relations of the kidney-ducts and the gonoducts, 

 and the presence of accessory or of vestigial organs in connexion 

 ^th them, will now be briefly dealt with. In the males of the 

 Bi^obranchs the mesonephric ducts which, as already pointed 

 out, act both as kidney-ducts and gonoducts, dilate posteriorly 

 to form a pair of vesiculae seminales, and then unite to form 

 a urinogenital sinus, opening into the cloaca at the extremity 

 of a median papilla (Fig. 230, C). The sinus also receives ducts 

 from the hinder part of the mesonephros, either separately, as in 

 the female, or by a common duct on each side — the so-called 

 metanephric duct — as in the male. Two tubular caecal out- 

 growths from the sinus form two sperm sacs. Only the anterior 

 portions of the Miillerian ducts with their coelomic apertures are 

 retained in the adult. In the female the mesonephric ducts are 

 purely excretory, but otherwise they are similar, and the oviducts 

 (Miillerian ducts) open into the cloaca separately or by a 

 common orifice (Fig. 230, D). A glandular dilatation of each 

 oviduct forms the oviducal or shell gland by which the horny 

 egg-cases are secreted. In the males of the Holocephali the 

 gonoducts open into a urinogenital sinus with an external orifice 

 distinct from and behind the anus ; but the female has separate 

 apertures for the rectum, the conjoined oviducts, and the 

 united mesonephric ducts. Both sexes have complete Miillerian 

 ducts communicating with the coelom in front, and behind with 

 the exterior. The Dipnoi of both sexes essentially resemble 

 the Elasmobranchs in the general relations of their ducts, but the 

 Miillerian ducts of the male exhibit marked differences in the 

 three genera.^ In Neoceratodus the ducts are as complete as their 

 functional representatives in the female. Protopterus retains 

 anterior vestiges and the coelomic apertures, and also vestiges of the 

 hinder portions which unite and end blindly in the urinogenital 

 papilla, but the middle sections of the two ducts are suppressed 

 (Fig. 233, B). In the Teleostomi there is a general similarity 

 in the terminal relations of the gonoducts and kidney-ducts. In 

 the Ganoids the archinephric ducts unite and then expand into 

 a urinary sinus or bladder, and the gonoducts of the female, or 

 of both sexes in Lepidosteus, open either into the archinephric 



^ Graham Kerr, op, cit. 



