THE URINOGENITAL ORGANS OF VERTEBRATES. 143 



organs 1 , one of which appears in each segment. If embryology is 

 in any way a repetition of ancestral history, it necessarily follows 

 that these tubes were primitively independent of each other. 

 Ancestral history, as recorded in development, is often, it is true, 

 abridged ; but it is clear that though abridgement might prevent 

 a series of primitively separate organs from appearing as such, 

 yet it would hardly be possible for a primitively compound 

 organ, which always retained this condition, to appear during 

 development as a series of separate ones. These considerations 

 appear to me to prove that the segmented ancestors of verte- 

 brates possessed a series of independent and segmental ex- 

 cretory organs. 



Both Professor Semper and myself, on discovering these 

 organs, were led to compare them and state our belief in their 

 identity with the so-called segmental organs of Annelids. 



This view has since been fairly generally accepted. The 

 segmental organs of annelids agree with those of vertebrates in 

 opening at one end into the body-cavity, but differ in the fact 

 that each also communicates with the exterior by an inde- 

 pendent opening, and that they are never connected with each 

 other. 



On the hypothesis of the identity of the vertebrate segmental 

 tubes with the annelid segmental organs, it becomes essential to 

 explain how the external openings of the former may have 

 become lost. 



This brings us at once to the origin of the segmental duct of 

 the kidneys, by which the secretion of all the segmental tubes 

 was carried to the exterior, and it appears to me that a right 

 understanding of the vertebrate urinogenital system depends 

 greatly upon a correct view of the origin of this duct. I would 

 venture to repeat the suggestion which I made in my original 

 paper (loc. cit.} that this duct is to be looked upon as the most 

 anterior of the segmental tubes which persist in vertebrates. 



1 Further study of my sections has shewn me that the initial independence of 

 these organs is even more complete than might be gathered from the description in 

 my paper (loc. cit.}. I now find, as I before conjectured, that they at first correspond 

 exactly with the muscle-plates, there being one for each muscle-plate. This can be 

 seen in the fresh embryos, but longitudinal sections shew it in an absolutely demon- 

 strable manner. 



