DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 67 



teloblasts and the rows of cells in front of them sink beneath 

 the ectoderm and come to form a separate and distinct layer 

 of their own, between the ectoderm on the outside and the 

 mesodermal lining of the coelomic cavities on the inside. 

 In longitudinal sections of young embryos (fig. 5) this layer 

 towards the posterior end gives the appearance of a kind of 

 string of nephridial cells. The large nephridial teloblast 

 together with a row of smaller cells lying in front of it form 

 the definitive nephridial layer. The transition from the previous 

 stage can be well appreciated by comparing the position of the 

 nephroblasts and the nephridial cells in figs. 2, 3, and 4, where 

 they are superficial, with the deeper position occupied by them 

 in figs. 5 and 7. This nephric cord is single-layered in the 

 beginning and remains so for a long time at the posterior end, 

 but its cells soon begin to multiply and proliferate opposite 

 and behind the intersegmental septa which divide one coelomic 

 sac from another, so that we get groups of these nephridial 

 cells situated at intersegmental intervals. The cells of these 

 intersegmental nephridial groups multiply here beneath the 

 peritoneal lining of the coelom, and the cells tend to travel 

 backwards towards the middle of the segment. Some of these 

 nephridial cells push their way into the septa between the two 

 apposing walls of the adjoining coelomic chambers (figs. 7 

 and 13). 



We thus get these intersegmental nephridial masses segregat- 

 ing into two separate groups, one keeping its ' retroperitoneal ' 

 position while shifting backwards and multiplying rapidly, 

 the other consisting of very few cells which make their way 

 into the septa and he between the two sheets of peritoneum 

 forming the two faces of the septa. In earher stages — or what 

 amounts to the same thing, in the posterior segments of the 

 embryos — we can see, in longitudinal sections, one nephridial 

 group in each segment lying at the posterior of the two angles 

 formed by the septum with the ventral body- wall (fig. 8). 

 In later stages, or in the more advanced anterior segments, 

 the segregation into two groups becomes quite evident. The 

 group consisting of a few cells caught in between the two septal 



