THE DEVELOPMENT OF PALUDINA VIVIPARA. 101 



and in the extreme original left and ventral (now right and 

 dorsal) corner of it is the left kidney {l.k.), contrasted with 

 the right in being much less developed, but in every other 

 respect perfectly comparable to it, and showing exactly the 

 same relations to both the pericardium and the original left 

 horn of the mantle cavity as it did before. Dorsally the 

 pericardium is narrowed to a point beside the liver, and here 

 a proliferation of cells is just beginning to take place, which 

 is the rudiment of the gonad (g.). 



The same structures are seen further advanced and more 

 highly magnified in figs. 2, 3, and 4, which are three nearly 

 adjacent transverse sections through a later stage. The 

 position of the organs in the body is quite similar to that 

 already seen, but here only the extreme (original) left-hand 

 corner of the pericardium (j)c.) is cut through. The rudi- 

 mentary kidney is seen at /. k., now showing a wide lumen, 

 but having only a narrow communication with the pericardium, 

 and the solid end of the duct is seen as before at l.m.c. 

 Fig. 4 shows the rudiment of the gonad {g.), now a well- 

 developed cord of cells, distinctly connected with the peri- 

 cardial epithelium; while fig. 3, a section intermediate 

 between figs. 2 and 4, just cuts through the edge of both 

 kidney (/. k.) and gonad (g.), and shows their close proximity. 

 This section is, however, chiefly interesting as showing the 

 thickening of the coelomic epithelium which connects these 

 two organs, and very soon becomes indistinguishable from 

 the gonad. A clear understanding of the position of these 

 rudiments is important, and will readily be obtained by a 

 comparison of the above-mentioned figures, especially of 

 figs. 1 and 15. From these it will be seen that whereas the 

 kidney is from the first on the original ventral side of the 

 pericardium, the gonad is a dorsal proliferation, which from 

 the time of its first formation lies close against the liver, the 

 proximity of gonad and kidney being merely due to the 

 extreme narrowness of the pericardium in this region. A 

 comparison of these figures with von Erlanger's (fig. 5, 

 pi. xxiii) seems to me to point to the conclusion that his 



