DISTOMUM CIRRIGERUM. 291 



The Relationship between the Size of Cell and the Size 



OF Animal. 



Ill studying tlie growth of the tissues and organs a 

 question arises which is seklom or never considered, as to the 

 relative sizes of the cells in a young and adult animal. Do 

 the cells maintain the same mean size in the young as in the 

 full-grown animal ? A priori it might be considered probable 

 that growth would imply simply an increase in the number of 

 cells, and that the mean size of the cells would remain con- 

 stant throughout. 



If, however, we compare Figs. XV and XVI, it can be seen 

 that the nuclei of the young animal have a diameter only 

 about a quarter that of the nuclei of the full-grown adult, also 

 in individuals which are larger but not full-grown the nuclei 

 are of an intermediate size. Hence during a certain 

 period of growth Ave may expect that the size of the cell is 

 a definite fraction of the size of the animal. 



As a rough test three encysted animals were taken, measur- 

 ing '70, "SO, and '17 mm. in diameter; the diameters of twelve 

 nuclei of epidermal cells were measured in each individual, 

 and the means were 11 /u, Q /m, and 3 fx respectively. Thus quite 

 clearly as the animal grows the nuclei become larger. 



Wishing to confirm this result on fresh material, some 

 fourteen Daphnia of varying sizes were taken at random 

 from a tank, and the maximum diameters of twelve consecu- 

 tive epidermal cells were approximately measured in each 

 individual by the following method. The surface of the 

 carapace is covered with a sculpturing consisting of regularly 

 arranged polygonal areas. In a carefully stained animal the 

 nuclei of the underlying epidermal cells can be seen, and as a 

 rule one nucleus occurs under the centre of each polygonal 

 area ; accordingly the polygonal areas correspond to cell out- 

 lines. Occasionally two nuclei can be seen under one of these 

 areas, but it is not of very frequent occurrence, and since it 

 was found as often in young as in older animals the fact that 



