DEVELOPMENT OF NEPHRIDIA OF PHBRBTIMA 79 



complete, and, on examination of the embryo under low 

 power, we can see the setae ; but in younger embryos, although 

 setal sacs and muscles can be made out in sections, the setae 

 are not yet formed and so cannot be distinguished externally. 



(a) Secondary Septal Nephridia. 



The secondary septal nephridia arise very much in the same 

 way as the primary septal pair. They appear ventral to the 

 primary septals and, like the latter, appear in pairs. It is 

 very difficult to say whether this paired origin is maintained 

 throughout the development of all the septal nephridia, but 

 it is certain that the first two secondary nephridia arise in 

 pairs. We may also note that these nephridia appear later 

 than the secondary integumentary nephridia, since in the 

 posterior segments of embryos with well-developed secondary 

 nephridia in their anterior portion, through we can make out 

 the rudiments of secondary integumentary nephridia, 

 the septal ones have not yet been formed. 



The group of cells forming a very early rudiment lies, as in 

 the case of the primary pair, between the two peritoneal 

 sheets of a septum and is consequently 'inter-peritoneal'. 

 One of the cells of this group is larger than the rest and corre- 

 sponds, in all probability, to the ' funnel-cell '. As regards the 

 original relations of this secondary pair we have to note, in 

 the first place, that their rudiments lie ventral to and at 

 some little distance from the primary nephridia, and, secondly, 

 that until the nephridium is almost fully formed and has 

 developed its long terminal duct there is no connexion between 

 this and the primary nephridium, nor are there any stray cells 

 lying on the septum between these two nephridia. The obvious 

 inference is that the secondary nephridia do not arise by a 

 process of budding or the like from the primary nephridia, 

 but do so de novo at their place of origin. Whence do the 

 rudiments of these nephridia come ? 



In describing the ultimate origin of the primary septal 

 nephridia we traced their beginnings to a group of cells which 

 pushed their way into each septum, and which, in their turn. 



