392 THE O RIG IX OF VERTEBRATES 



alive the belief in the origin of vertebrates from a segmented annelid. 

 These segmental organs thus compared were the mesonephric tubules, 

 and doubts arose, especially in the mind of Gegenbaur, as to the 

 validity of such a comparison, because the mesonephric tubules did 

 not open to the exterior, but into a duct — the segmental duct — which 

 was an unseginented structure opening into the cloaca ; also because 

 the segmental duct, which was the excretory duct of the pronephros, 

 was formed first, and the mesonephric tubules only opened into 

 it after it was fully formed. Further, the pronephros was said to 

 arise from an outbulging of the somatopleuric mesoblast, which 

 extended over a limited number of metameres, and was not segmental, 

 but continuous. Gegenbaur and others therefore argued that the 

 original prevertebrate excretory organ was the pronephros and its duct, 

 not the mesonephros, from which they concluded that the vertebrate 

 must have been derived from an unseginented type of animal, and 

 not from the segmented annelid type. 



Such a view, however, has no further reason for acceptance, as 

 it was based on wrong premises, for Euckert has shown that the 

 pronephros does arise as a series of segmental nephric tubules, and 

 is not unsegmented. He also has pointed out that in Torpedo the 

 anterior part of the pronephric duct shows indications of being seg- 

 mented, a statement fully borne out by the researches of Maas on 

 Myxine, who gives the clearest evidence that in this animal the 

 anterior part of the pronephric duct is formed by the fusion of a 

 series of separate ducts, each of which in all probability once 

 opened out separately to the exterior. 



Euckert therefore concludes that Balfour and Semper were right 

 in deriving the segmental organs of vertebrates from those of annelids, 

 but that the annelid organs are represented in the vertebrate, not by 

 the mesonephric tubules, but by the pronephric tubules and their 

 ducts, which originally opened separately to the exterior. By the 

 fusion of such tubules the anterior part of the segmental duct was 

 formed, while its posterior part either arose by a later ccenogenetic 

 lengthening, or is the only remnant of a series of pronephric tubules 

 which originally extended the whole length of the body, as suggested 

 also by Maas and Boveri. Euckert therefore supposed that the 

 mesonephric tubules were a secondary set of nephric organs, which 

 were not necessarily directly derived from the annelid nephric 

 organs. 



