l6o THE URINOGENITAL ORGANS OF VERTEBRATES. 



ova are carried off in both by a specialized oviduct. The 

 Wolffian duct, or ureter, is found both in Selachians and Am- 

 phibians, and the relations of the testis to it are the same in 

 both, the vasa efferentia of the testes having in both the same 

 anatomical peculiarities. 



The following points are the main ones in which Selachians 

 and Amphibians differ as to the anatomy of the urinogenital 

 organs ; and in all but one of these, the organs of the Amphi- 

 bian exhibit a less differentiated condition than do those of the 

 Selachian. 



(1) A glandular portion (Kopfniere) belonging to the first 

 segmental organ (segmental duct of the kidneys) is found in all 

 embryo Amphibians, but usually disappears, or only leaves a 

 remnant in the adult. It has not yet been found in any Se- 

 lachian. 



(2) The division of the primitive duct of the kidney into 

 the Miillerian duct and the Wolffian duct is not completed so far 

 in Amphibians as Selachians, and in the former the two ducts 

 are confluent at their lower ends. 



(3) The permanent kidney exhibits in Amphibians no 

 distinction into two glands (foreshadowing the Wolffian bodies 

 and true kidneys of higher vertebrates), as it does in the Se- 

 lachians. 



(4) The Miillerian duct persists in its entirety in male Am- 

 phibians, but only its upper end remains in male Selachians. 



(5) The openings of the segmental tubes into the body- 

 cavity correspond in number with the vertebral segments in 

 most Selachians, but are far more numerous than these in 

 Amphibians. This is the chief point in which the Amphibian 

 kidney is more differentiated than the Selachian. 



The modifications in development which the urinogenital 

 system has suffered in higher vertebrates (Sauropsida and 

 Mammalia) are very considerable ; nevertheless it appears to 

 me to be possible with fair certainty to trace out the rela- 

 tionship of its various parts in them to those found in the 

 Ichthyopsida. The development of urinogenital organs has 

 been far more fully worked out for the bird than for any other 

 member of the amniotic vertebrates ; but, as far as we know, 



