60 KARM NARAYAN BAHL 



These strings of nophridial colls aggregate later into groups 

 that lie opposite and a little posterior to the places where the 

 intersegmental septa join the body-wall. These groups of 

 cells, situated underneath the mesodermal peritoneal membrane 

 (' somatopleure '), proliferate to form masses of cells, which, 

 as they grow, begin to project into the coelomic cavity. These 

 constitute the nephridial rudiments. They carry with them 

 in their growth the sheet of peritoneal membrane, which now 

 forms an enveloping sheath over these unformed nephridia 

 (fig. 9). In longitudinal sections of an embryo, about 4 mm. 

 in length, these embryonic nephridia are seen for the most 

 part as solid clup-shaped masses, lying in the anterior part of 

 each coelomic chamber, a pah' in each segment of the body 

 except the first two. While the first two segments are devoid 

 of nephridia and the greater part of the embryo possesses sohd 

 nephridial masses, some of the anterior segments (seventh and 

 eighth, for example) have fully-formed nephridia with the charac- 

 teristic shape and the intra-cellular canals of the adult organ. 



In preparations of whole embryos of suitable age, flattened 

 after opening them through the mid-dorsal line, we can see 

 the rudiments of these primary nephridia as elongated masses 

 lying posterior to the septa towards the hind end of the 

 embryo ; but, as we examine the segments in front, we get 

 the nephridia in all stages of development in the same embryo, 

 since development proceeds antero-posteriorly. We may note 

 here that these nephridia have no connexion with the septal 

 partitions, and consequently a ' septal funnel ' is never formed 

 at any stage of development of this primary pair of integu- 

 mentary nephridia. 



At this stage of development (4 mm. long) the embryo 

 exhibits a typical meganephric or paired condition like that 

 of the adult Lumbricus, having a pair of ' true ' ectodermal 

 nephridia in each segment (Text-fig. 1 a). This marks the 

 first stage in the development of nephridia in Pheretima, 

 which comprises the developmental history from the first 

 appearance of teloblasts up to the formation of a pair of primary 

 integumentary nephridia in each segment. 



