1904.] M0NTG0MP:RY — MORPHOLOGICAL SUPERIORITY. 375 



testis. Reduction of one member of a structural pair is always a 

 departure from the primitive condition, hence a more advanced 

 morphological state than retention of both members. But with the 

 case in point this morphological difference cannot be justly thought 

 of much value, for with the right ovary of Birds, as with one of the 

 lungs of Snakes, the reduction is due simply to mechanical pressure 

 inducing stoppage of the nutritive blood fluid, and not to any pro- 

 found change in the growth processes. 



Turning to the comparison of the genital ducts, we find the most 

 primitive condition in the Cychsfomata, certain Teleostei, and, 

 according to Gegenbaur, in the Selachian Lcemargus : there are no 

 genital ducts, but the germ cells fall directly into the body cavity 

 (coelom), and are discharged to the exterior through abdominal 

 pores. In the Acrania there are also no ducts, but the relations are 

 less simple in that the genital products fall into the atrium (an 

 ectodermic cavity) and from there are passed to the exterior 

 through the atriopore. In these forms the sexes are alike in the 

 mode of discharge of the genital products. 



In all other Vertebrates there are special genital ducts, which 

 may be considered separately for each sex. 



Four kinds of genital ducts may be distinguished in the male, 

 according to their mode of embryonic formation, (i) The seg- 

 mental duct (pronephric duct, duct of the earliest kidney system, 

 the pronephros) persists as the urogenital duct, as the common 

 duct of testes and kidneys. This is the most usual condition, and is 

 found in all the Amniota (Reptiles, Birds, Mammals) and Batra- 

 chia, all the Selachii except LcBmargus, in Lepidosteus and Aci- 

 penser. In all these cases a Miillerian duct is laid down, but either 

 remains embryonic or disappears more or less completely. (2) A 

 direct backward growth of the testis itself is the genital duct : most 

 Teleostei. (3) An open groove of the peritoneum serves as a geni- 

 tal duct : Polypterus and Amia, according to the description by 

 Jungersen.^ (4) In Protoptems a tube formed within the testis is 

 the sperm-duct, and this unites with the persistiig posterior end of 

 what is considered a Miillerian duct.^ Of these kind of ducts the 

 last three are much more alike among themselves than is any of 

 them to the first kind. It follows that for all gnathostomatous 

 Vertebrates, except some Teleostomi Lcemargus and Proiopterus, 



"^ Zoolcg. Anzeiger, 1900. 



2 According to the account by W. N. Parker. 



