H. M. FucHS 221 



II. Method.s. Ijiportance of Exactness. 



Before detailing the technique employed, the essential nature of 

 the method of experimentation should be explained. Shortly put, it 

 was as follows. Approximately equal numbers of eggs from a single 

 animal were placed in two or more dishes, each of which contained 

 exactly the same quantity of sea-water. Fertilization was then effected 

 by the addition to each dish of exactly the same amount of sperm- 

 suspension from a single animal. The object was to discover the effect 

 on the eggs of different treatments of those in the different dishes, or 

 the effects on the spermatozoa of varj'ing preliminary treatments of the 

 different .samples to be added to the dishes. If this preliminary handling 

 consisted in adding a certain solution to the quantity of sperm-sus- 

 pension to be used for fertilizing the eggs in one of the dishes, the 

 consequent dilution of the suspension was compensated for by the 

 addition to the other lots of sperm of equal quantities of plain sea- 

 water. The criterion of the effects on the spermatozoa of this preliminary 

 handling consisted in a comparison of the jjercentages of eggs fertilized 

 when the differently treated but equal quantities of sperm-suspension 

 were added to the dishes containing the eggs. 



From this outline a number of absolutely necessary precautions can 

 be deduced : 



(1) If the experiment consists in the comparison of the effects of 

 a different treatment either of the eggs or of the spermatozoa, the eggs 

 in the different dishes must all be derived from one individual, or no 

 legitimate comparison can be made. Similarly, the sperm used to effect 

 the fertilizations must all be derived from one animal, and moreover 

 the definite quantities used must all be drawn from the same sus- 

 pension. The reason for this is that it is found to be impossible in 

 practice to make up two sperm-suspensions of equal concentrations. 

 The only method of judging the concentration of .spermatozoa in a 

 suspension is by the degree of milkiness, as seen with the eye. Differ- 

 ences in the degree of milkine.ss which are undetectable by the eye 

 may mean considerable variation in the concentration of the sperma- 

 tozoa (as measured by the percentages of eggs fertilized by equal 

 quantities of the different suspensions). 



(2) An experiment can be made to compare the proportions of 

 eggs fertilized when equal numbers are taken from different indivi- 

 duals, and to each lot is added an exactly equal amount of one sperm- 



