THE URTNOGEXITAL ORGANS OF VERTEBRATES. 145 



series of transverse tracheal tubes affords a parallel to the forma- 

 tion of a duct from the coalescence of a series of segmental 

 tubes. 



Though it must be admitted that the loss of the external 

 openings of the segmental organs requires further working out, 

 yet the difficulties involved in their disappearance are not so 

 great as to render it improbable that the vertebrate segmental 

 organs are descended from typical annelidan ones. 



The primitive vertebrate condition, then, is probably that of 

 an early stage of Selachian development while there is as yet 

 a segmental duct, the original foremost segmental tube open- 

 ing in front into the body-cavity and behind into the cloaca ; 

 with which duct all the segmental tubes communicate. Vide 

 Fig. 2. 



The next condition is to be looked upon as an indirect 

 result of the segmental duct serving as well for the products 

 of the generative organs as the secretions of the segmental tubes. 



As a consequence of this, the segmental duct became split 

 into a ventral portion, which served alone for the ova, and 

 a dorsal portion which received the secretion of the segmental 

 tubes. The lower portion, which we have called the oviduct, 

 in some cases may also have received the semen as well as 

 the ova. This is very possibly the case with Ceratodus (vide 

 Gunther, Trans, of Royal Society, 1871), and the majority of 

 Ganoids (Hyrtl, Dcnkschriften Wien, Vol. VIII.). In the majo- 

 rity of other cases the oviduct exists in the male in a completely 

 rudimentary form ; and the semen is carried away by the same 

 duct as the urine. 



In Selachians the transportation of the semen from the 

 testis to the Wolffian duct is effected by the junction of the 

 open ends of two or three or more segmental tubes with the 

 testicular follicles, and the modes in which this junction is 

 effected in the higher vertebrates seem to be derivatives from 

 this. If the views here expressed are correct it is by a complete 

 change of function that the oviduct has come to perform its 

 present office. And in the bird and higher vertebrates no trace, 

 or only the very slightest (vide p. 165) of the primitive urinary 

 function is retained during embryonic or adult life. 



The last feature in the anatomy of the Selachians which 

 B. 10 



