J go JV/LSOJV. [Vol. I. 



segmental duct in Amphibia (Rana) and Reptilia (^Lacerta) is 

 given in a brief note just published by Perenyi.^ 



Now, in some cases at any rate, the segmental tubules of the 

 vertebrate pronephros are formed wholly or in part as out- 

 growths of the segmental duct, and thus agree precisely in mode 

 of origin with the nephridia of Lumbrictis. As far, therefore, 

 as exact likeness in the development of special parts can be 

 taken to indicate homology, the " nephridial row''' of Lumbricus 

 must be regarded as homologous with the segmental duct, and the 

 series of nephridia as homologous with the vertebrate pronephros^ 



It must, however, still remain an open question how far the 

 comparison between the nephridia of annelides and those of ver- 

 tebrates can be carried out until it is ascertained how far these 

 structures are serially homologous with one another. Van Wijhe's 

 positive statements are strongly confirmatory of tlie ordinary 

 view that the mesonephric tubules are of mesoblastic origin, 

 and only secondarily become joined to the ectoblastic segmental 

 duct, so that they would seem not to be serially homologous with 

 the pronephric tubules. We have seen, however, that the anne- 

 lidan nephridia consist of a proximal mesoblastic portion (fun- 

 nel) and a distal ectoblastic portion. If we suppose these two 

 portions to have existed in all the nephridial tubules of the 

 common ancestral type, and to have subsequently varied in im- 

 portance in different regions of the body, one or the other por- 

 tion increasing at the expense of the other, the difficulty of 

 understanding the serial homologies disappears. 



Turning now to more special questions, we find a likeness 

 between the germ-bands of Lumbricus and those of Clepsine so 

 great as to indicate a very near relationship between the Oligo- 

 chaeta and the Hirudinea. This resemblance can hardly be 

 adaptive merely, since Lumbricus and Clepsine differ very widely 

 in respect to the mode of gastrulation, and there is nothing in the 

 conditions of larval life to suggest an explanation of the likeness 

 in the germ-bands. Probably no one will question the homology 

 of the neuroblasts in the two animals. The four nephroblasts of 



' Zoologischer Anzeiger, No. 243, 1887. 



''The validity of the comparison is not affected by the absence of special nephoblasts 

 in the vertebrates, for these cells have arisen in Lumbricus and Clepsine simply through 

 extreme concentration of development, and represent the entire proliferating group in 

 the vertebrates. 



