DEVELOPMENT OP NEPHRIDIA OF PHERETIMA 53 



The question as to which category (epiblastic or mesoblastic) 

 the Ohgochaete nephridia belong has recently been attacked 

 by Staff (15), by renewed researches into the mode of their 

 development in Criodrilus. 



Staff found ' that in Criodrilus lacuum the mother- 

 cells of the nephridia appear in the ectoderm at the hinder 

 region of the embryo, and here act as teloblasts, giving 

 rise to strings of cells by continuous budding off of smaller 

 cells in front of them, like the mesodermic teloblasts situated 

 internally to them. There are on each side four rows of such 

 ectodermal teloblasts, and the rows of cells to which they give 

 rise become wedged in between the ectoderm and the coelomic 

 mesoderm. The strings of cells destined to give rise to the 

 nephridia are broken into groups, and one group is pushed into 

 each septum which divides one coelomic sac from another. 

 Here each group grows and gives rise to a chain of cells, and 

 this cell-chain becomes hollowed out and forms a tube. Its 

 most internal cell projects into the coelomic cavity between the 

 coelomic cells forming one side of the septum, and forms 

 the greater part of the coelomic funnel of the nephridium. The 

 lower lip of the funnel is constituted by one huge cell belonging 

 to the coelomic wall ' (12). 



According to Staff, therefore, the nephridia develop from 

 the ' retroperitoneal ' cell-row, lying lateralwards to ' primitive 

 muscle-fibres ' in the manner that this breaks up in segmentally- 

 arranged cell-groups, which project into the body-cavity and 

 are covered over with the peritoneum. The whole nephridium 

 is really ectodermal. The result of Staff's investigation, 

 therefore, is to uphold Goodrich's view. 



The earthworm Pheretima (Perichaeta), the develop- 

 ment of which I have studied for the purpose of this paper, 

 has all along been held to possess a branched ' plectonephric ' 

 nephridial system, a term which has become inapplicable to 

 the system in Pheretima on our further knowledge of it 

 gained recently (1). The development of this latter type of 

 nephridia has been investigated by Beddard (3) in Octo- 

 chaetus multiporus, by Vejdovsky (16) in Mega- 



