392 THE ORIGIN 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 unsegmented 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 unsegmented 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 Rickert 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. 
Rickert 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. Rickert therefore supposed that the 
mesonephric tubules were a secondary set of nephric organs, which 
were not necessarily directly derived from the annelid nephric 
organs. 
