68 KARM NARAYAN BAHL 



sheets, leads, to anticipate matters, to the development of 

 septal nephridia, which we shall speak of in the next part of 

 the paper, while the other group consisting of a numher of cells 

 lying beneath the peritoneum and immediately posterior to 

 each intersegmental septum, is the rudiment of the primary 

 pair of integumentary nephridia. We shall now consider the 

 details of development of these integumentary nephridia. 



This group of cells beneath the peritoneum is the ' retro- 

 peritoneal ' group of Meyer (13) and forms the forecast of the 

 whole primary nephridium of the first stage. The cells of this 

 group separate away from the septum, divide and proliferate 

 so as to bulge out as sohd masses into the coelomic cavities, 

 as shown in fig. 8, c. They carry with them their peritoneal 

 covering which forms a thin sheath round these solid nephridia. 

 The growth is not only vertical but also horizontal, and the 

 nephridial rudiment besides increasing in thickness and pro- 

 jecting into the coelom also extends laterally, so that in 

 a preparation showing the body-wall of an embryo flattened 

 we get a pair of deeply-staining elongated solid masses of cells 

 lying immediately behind each septum as shown in fig. 11, a, 

 and Text-fig. 4. 



By what steps this elongated ridge lying behind each 

 coelomic septum develops into an adult nephridium I have 

 shown in fig. 11 (a-f). The earlier stages, in which the nephro- 

 blasts and their derivatives multiply, form masses of cells at 

 septal places which segregate further into two groups, a smaller 

 one, the cells of which push their w^ay into the intersegmental 

 septa, and a larger one, the cells of which move backwards 

 and form the so-called ' retroperitoneal ' group of cells, which 

 forms the elongated solid ridge bulging into the coelomic 

 cavity, can all be followed in a few series of longitudinal and 

 transverse sections ; but once the nephridial rudiment has 

 reached the size and shape of the elongated mass, shown in 

 fig. 11 A, we can follow its further development best in whole 

 embryos that have been opened in the mid-dorsal Hne, their 

 endoderm with food-yolk removed, and the remaining portion 

 mounted flat. 



