306 THE EFFECT OF FOOD 



three times daily, and the constant slow interchange 

 through the muslin, were insufficient to equalise the 

 proportions of metabolic products in the two vessels for 

 more than a short time, so that on an average the water 

 in the inner vessel was more foul than that in the 

 outer. This fouling would probably be much increased 

 by particles of decomposing vegetable matter and of 

 animal excreta collecting in the fibres of the muslin 

 and on the inner walls of the glass tube, and constantly 

 poisoning the water. The outer water would also be 

 fouled in this manner, but to a very much slighter ex- 

 tent, for the " fouling area " of muslin and walls of ves- 

 sels would be proportionately very much less. That 

 the metabolic products from unhealthy or decomposing 

 vegetable matter can exert a most harmful influence on 

 growth is shown by some of my own experiments with 

 plutei. Thus ova allowed to develop in water which 

 had previously contained 1 or 2 gm. per litre of (pre- 

 sumably unhealthy) seaweed, were diminished in size 

 by as much as 13.2 and 18.1 per cent.* 



De Varigny's experiments on the influence of super- 

 ficial area of water must be considered in conjunction 

 with some observations by Yung f on tadpoles. Yung 

 put twenty-five freshly hatched tadpoles in each of 

 three vessels which contained equal volumes of water 

 (1200 cc.), but of which the diameters were respect- 

 ively 7 cm., 11 cm., and 14.5 cm. Thus the surface of 

 water exposed to the air varied in the proportions of 

 1 : 2.5 : 4.3 After a month and a half the tadpoles 

 were found to have attained the following average sizes : 



* Mittheihmgen a. d. Zool. Stat. z. Neapel., Bd. xiii. p. 348. 

 t Arch, des Sci. Phys. et Nat., xiv. p. 502, 1885. 



