DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 77 



The first is the inter-peritoneal situation of the rudiment of 

 a septal nephridium, i. e. in other words, the septal nephridia 

 are not derived from one of the cells belonging to the peritoneal 

 lining of the septa but take their origin from cells lying between 

 the two sheets of peritoneum forming a septum. The second 

 is that these rudiments can be traced directly to the original 

 nephric row. Since the original nephric row is ectodermal in 

 origin we have established the ectodermal origin, in the last 

 analysis, of the septal nephridia. 



In all descriptions of previous work on the development of 

 nephridia in earthworms, mention is made of a ' funnel-cell ' 

 (' Trichterzelle '), a term which is used in at least two senses. 

 It is either used for a single large cell which is separated off 

 very early from the nephric row and forms the forecast of the 

 whole nephridium, or it is used for the most internal cell of 

 a series or group of cells which go to form the whole nephridium, 

 and, in this case, the ' funnel-cell ' gives rise only to the funnel 

 of the nephridium. The term is used in the former sense by 

 Bergh in the case of Criodrilus and Lumbricus (6 and 7), 

 and in the latter sense by Staff in the case of Criodrilus (15) . 



During the development of nephridia in Pheretima 

 also we can distinguish a large cell which is probably the equiva- 

 lent of the ' funnel-cell '. So far as the development of the 

 integumentary nephridia are concerned, the ' retroperitoneal ' 

 group of cells, which give rise to them, contains no ' funnel- 

 cell ' in it, nor, as we have seen, do we get a funnel formed in 

 the adult integumentary nephridia. But with regard to the 

 septal nephridia we can distinguish a cell larger than others 

 at almost all stages of their development. In fig. 6, which 

 represents part of a transverse section of the posterior end of 

 a young embryo, we can distinguish one large cell in connexion 

 with the septum on each side. Further, during the passage of 

 this cell to its dorso-lateral position, one cell can always be 

 distinguished by its very large size as compared with the 

 surrounding peritoneal cells. Finally, when the rudiment of 

 the septal nephridium consist of a group of three or four 

 cells lying on the septum on each side of the dorsal vessel, 



G 2 



