554 PEOP. G. B. HOWES ON SOME 



the urino-genital sinus of the Marsipobrancli is a hypoblastic 

 sac, split off from the enteric tube, the difficulty of homolo- 

 gizing the perforations of its side walls with the cloacal pits 

 of the other Pisces, which are epiblastic in origin, will be still 

 further increased. With this the mind reverts to Eathke's 

 original view *, shared by Gegenbaur and others, that the 

 Marsipobranchii should be included in the category of those 

 fishes which have lost their genital ducts. 



Inasmuch as their ducts are absent in both sexes, they may 

 be said to be in a gymnogonarial condition, as distinguished from 

 the gymnoarian Osteichthyes, in which the females only are 

 ductless. 



The chief obstacle in the way of accepting Rathke's view has 

 undoubtedly been the failure of embryologists to find traces 

 either of a splitting of the segmental duct or of parorchis and 

 parovarium in the young of these fishes. If the view of the 

 phylogeuy of the Teleostean "ovary-duct" which I have sought 

 to establish should remain valid, this objection will have been 

 largely dispensed with; for, inasmuch as the Marsipobranchs, 

 together with the Salmones, Murcenidce, Galaxiidce, Notoptericlce, 

 Hyodon, and Colitis, Avould appear to have lost not the Miillerian 

 duct but the primitive and hermaphroditic " ovary-duct," the 

 demand for vestiges of the former will be no longer a sine qua 

 noil. 



Balfour has long ago pointed out f that " the condition of the 

 urino-genital organs in Selachians is by no means the most primi- 

 tive found amongst Vertebrates." Powerful arguments in favour 

 of a belief in convergence of the living Marsipobranchii, Granoidei, 

 and Teleostei towards a common stock, unrepresented at the 

 present day, have been lately put forward % by Beard. Edinger 

 has shown the structure of the prosencephalon of the Batoidei to 

 be more primitive than that of the Sharks, and 1 have been 

 enabled to prove § that, with respect to its dorsal mesentery, the 

 Torpedo Hypnos siibnigrum is far more primitive than all other 

 Plagiostomes. The latest palseontological researches of Cope|| and 



* Log. cit. p. 123. 



t Journ. Anat. & Phys. vol. x. p. 28 (1876). 



X Anat. Anz. 1890, pp. 146 & 179. 



§ Proe. Zool. Soc. Loud. 1890, p. 671. 



II Amer. Nat. vol. xxiy. p. 402 (1890). 



