22'2 Studies in the PhyHiolo^m of Fertilization 



suspension. The reciprocal experiment of fertilizing equal quantities 

 of eggs from one animal with equal amounts of sperm-suspensions from 

 several other individuals does not give a valid result. This again is 

 owing to the fact that it is impossible to make up different sperm- 

 suspensions of exactly equal concentrations. 



(3) The way in which the spermatozoa are subjected to the dif- 

 ferent treatments is as follows. A suspension is made up and afterwards 

 divided among two or more dishes, which can then be subjected to 

 different conditions of temperature, etc., or be kept for different lengths 

 of time before being used. If the treatment consists in the addition of 

 some substance, equal amounts of the one suspension are tried out into 

 several dishes, and to each dish is added an equal volume of liquid con- 

 taining the substances. One dish is kept as a control of untreated 

 spermatozoa, and to this is added a like volume of plain sea-water. In 

 this way the concentrations of spermatozoa are kept identical in the 

 various subdivisions of the suspension. 



(4) In making the fertilizations in the different dishes of an 

 experiment, it is essential that all the eggs in each lot should have 

 an equal chance of being fertilized by the spermatozoa which are added. 

 This is brought about by pouring the water containing the eggs and 

 spermatozoa from one dish to another immediately after the sperm has 

 been added. This pouring was usually done ten times backwards and 

 forwards for each lot of eggs, which ensures a thorough mixing of the 

 water in which the eggs were lying with the sperm-suspension. The 

 definite quantities of the latter were measured out with a graduated 

 pipette, or by a certain number of drops from a pipette of uniform bore 

 closed at the top by the thumb. 



(5) Since the experiments consisted in comparing the percentages 

 of eggs fertilized under different conditions, it is obvious that these 

 percentages must be above and below 100. If none or if all of the 

 eggs are fertilized in each of the dishes, no comparisons can be made. 

 In consequence, the concentration of the sperm-suspension employed 

 must be such that some but not all of the eggs are fertilized. Now 

 this is a difficult matter to arrange, since the amount of dilution 

 necessary in making up a sperm-suspension so that some only of the 

 eggs will be fertilized when a definite (]uantity of the suspension is 

 added to them must be judged by the eye. In practice this can be 

 done with more or less success, but very often the experiment turns 

 out a failurt'. More than twice as many experiments were made as 

 are recorded below. All those not detailed were failures because the 



