8 E. J. LUND 



MEASUREMENT OF THE AMOUNT OF FOOD EATEN AND .METHOD 

 OF EXPERIMENTATION 



In order to express quantitative!}' the relations of Bursaria 

 to food, it is necessary to obtain a reliable method for measuring 

 the amount of food taken in a given length of time, under given 

 conditions. The unit of ^'olume employed was that of one 

 grain of fresh hard-boiled yolk of hen's egg. The eggs were 

 boiled fifteen minutes. These grains are readily eaten by Bur- 

 saria and may be obtained of an approximateh' uniform size. 

 It is necessary to deal with suspensions of such grains, having 

 a uniform concentration (that is, containing the same number 

 of grains to a given volume). For this purpose stock suspen- 

 sions were made up on successive days from yolk of the same 

 egg: these were made uniform by making them up in vials of 

 the same size and comparing each with a standard concentration 

 kept in a vial of the same size. Various known grades of con- 

 centration were then made up b}' adding a known volume of 

 the stock suspension to 5 cc. of water in a stender dish of 8 cc. 

 capacity. This procedure was found to be sufficiently accurate 

 to avoid the introduction of any observable variation in the 

 amount of j^olk eaten in a measured period of time (page 20.-) 

 Uniformity in the size of the yolk grains was obtained by repeat- 

 edly washing the fresh hard boiled yolk crystals in distilled or 

 tap water and decanting, until the suspension when left to settle 

 leaves a clear supernatant liquid. The smaller grains remain 

 in suspension a little longer than the larger ones and thus may 

 be removed b}- decantation. Uniformity in size is still further 

 obtained b}' drawing off the grains from the same level in the 

 clear suspension with a pipette. Some eggs have yolk crystals 

 of more uniform size than others, so that only the eggs best in 

 this respect have been used. 



^ In most of the experiments it was necessary to make up only a single stock 

 suspension, since the animals were fed only once and all the feeding was carried 

 out at the same time. In the case of experiments which required the feeding of 

 yolk on more than one day, however, this standard concentration had likewise 

 to be made up anew each day by comparison with that of the day before. 



