DISTOMUM CIRRIGERUM. 298 



Mean of twelve cells from the middle 

 Length of Leptodora of the proctodseum, 



in millimetres. Diameter of Diameter of 



cell. nucleus, 



/t fi 



1-3 ... .V4 ... 2-9 



3-0 ... 11-1 ... 4-5 



7-7 ... 14-4 ... 5-8 



Very probably, in some cases, the size of cells diminishes 

 as the body grows ; for example, the red blood-corpuscles of 

 large tadpoles appear to be somewhat smaller than those of 

 small tadpoles. 



These few examples which have been brought forward 

 would seem to indicate that the mean sizes of the cells are 

 some definite fraction of the size of the body, and that accord- 

 ing to the growth of the body as a whole, so these means may 

 either rise or fall. If such a law of growth be established it 

 lessens our appreciation of the individuality and relative 

 independence of the cell units, and it would weaken the view 

 which regards the body as a cell-republic. A thorough 

 statistical investigation into the subject would be of much 

 interest, and the results might lead to a great modification 

 of the current views as to the profound importance of the 

 cell. In a future publication I hope to enter into greater 

 detail. 



SUMMAEY. 



The material available for the present investigation was 

 quite inadequate for elucidating the life-history of the 

 species. The only new fact added is the discovery that the 

 sexual form can develop from the egg in the crayfish itself; 

 hence, were it not for the problematical D. isostomum we 

 should regard the trematode as monogenetic, and having a 

 quite simple life-histor}-. The formation of a cercaria-cyst 

 would indicate, however, that this monogenetic character was 

 secondarily acquired, and that at some former time it was 

 typically digenetic. 



