100 ISABELLA M. DKUMMOND. 



corresponding position on the wall of the narrow left-hand 

 chamber of the pericardium is seen the little left kidney {I. h.), 

 much less developed than its fellow on the right. In the 

 next section of the series the solid ends of the two horns of 

 the mantle cavity are found abutting each against the 

 kidney of its respective side. It is probably at about this 

 stage of development that von Erlanger describes the first 

 appearance of the retrogressive development of the primitive 

 left kidney. It is^ indeed^ extremely rudimentary at this 

 time, and might easily be overlooked, but I cannot find that 

 it ever wholly disappears ; rather it might be said that its 

 growth is arrested for a time, but at a slightly later stage it 

 again resumes its development. Still less can I find traces 

 of real retrogression in the primitive left horn of the mantle 

 cavity. It ceases to grow, or at least does not grow nearly 

 as rapidlj' as the primitive right horn which is to form the 

 kidney duct, but it always retains its original relation to the 

 pericardium, with its solid end abutting against the primitive 

 left ventral corner, and is never, as von Erlanger both 

 describes and figures, separated from it by a considerable 

 space (see his pi. xxi, figs. 12 and 13). 



As far as can be judged from the relative positions of the 

 organs, the embryo from which his fig. 12, pi. xxi, is taken 

 corresponds almost exactly to my fig. F in the drawings of 

 the whole animal. A section across the visceral hump of an 

 embryo of this stage is depicted in fig. 15. and shows how 

 the little left kidney, far from having disappeared, as 

 von Erlanger describes, is now larger than at the stage when 

 he believed it to be most fully developed. This figure is 

 wholly comparable with fig. 1, except for the change in the 

 position of the organs due to torsion. The heart, now fully 

 differentiated, is seen at It. in the same position relatively to 

 the other organs as the similarly marked mass of cells in 

 fig. 1, and the original right kidney, the definitive kidney of 

 the adult, is cut across at A-., with its duct adjacent to it at 

 h. (1. The original left portion of the pericardium is even 

 more narrow relatively to the right than is the case in fig. 1, 



