98 ISABELLA M. DRUMMOND. 



obtained with those of the authors above mentioned. In 

 Part II, therefore, I give a brief account of the development, 

 aiming not so much at a description of the organogeny, 

 except in one or two cases where my results differ from those 

 of von Erlanger, as at making clear the changes in position 

 and relative proportions of the organs in successive stages. 



Before concluding this introductory note it is conve- 

 nient to say a few words with regard to the plates. The 

 outlines of the figures of whole embryos have all been drawn 

 with a camera lucida from preparations of tl\e whole animal, 

 which is represented as transparent, the organs being shown 

 by a dotted line. These have, in fact, been also traced, 

 where possible, from whole preparations, but the tracings 

 thus obtained have been added to after a careful study of 

 sections. Figs. 11 to 17 explain themselves; they are for 

 the most part transverse sections through the visceral hump, 

 taken as far as possible through corresponding regions in 

 successive stages, and all orientated the same way on the 

 page, that is, as if the creeping sole of the foot were parallel 

 with the bottom edge, in order to facilitate coniparisons. 

 All are taken looking from behind forwards, — that is, the left 

 side of the figure is also the animal's left side. 



Part I. — The Urinogenital Organs. 



Von Erlauger (5 and 6), in his account of the developing 

 ccclom and its derivatives in Paludina, made known for the 

 first time the existence of the rudimentary original left 

 kidney, and showed conclusively that the existing kidney of 

 the Prosobranchs corres})onds to the definitive left kidney of 

 other Gasteropods. Moreover, he brought the Prosobranchs 

 much more closely into line with other Molluscs than had 

 hitherto been the case, by describing the gonad as a deriva- 

 tive of the pericardium, and as discharging its products 

 through a duct which was probably the duct belonging to 

 the rudimentary kidney. While entirely agreeing to this 

 extent with his results, 1 have arrived at conclusions with 



