'224 Sfiidiefi in the Plniniohxpi of FeHilization 



(b) Thoroiigli niLiiiKj of the eggs before dividing tlteiii, into differetd 

 lots. 



The ni'cessity for this precaution will be obvious in the light of 

 exjieriments to be described below, which show that eggs taken from 

 different parts of the oviduct of an individual Cionn have very different 

 capacities of being fertilized. 



(c) The importance of the way in xvhicli iiiseininatiuii is effected 

 has already been dwelt upon. A rapid and even distribution of the 

 spermatozoa through the wa,ter containing the eggs is essential, so that 

 all the eggs in the different dishes may have an equal chance of being 

 fertilized. The most accurate method was found to be as follows. A 

 certain amount of the sperm-suspension to be used was pipetted into 

 each of a number of dishes containing equal amounts (10 — 20 cc.) of 

 sea- water. The spermatozoa were thoroughly mi.xed with the water in 

 each of these dishes by repeated jjouring, after which the contents of 

 the dishes were poured on to equal amounts of eggs lying in a drop 

 or two of water in other vessels. The operation was finished by re- 

 pouring each of the lots a number of times. This was the method 

 adopted in most of the experiments, although sometimes the definite 

 amounts of sperm-suspension were simply pipetted straight into dishes 

 in which the eggs were lying in equal quantities of water. After this 

 the liquids were, of course, poured backwards and forwards several times 

 to ensure thorough mixing. 



(d) The cownting of the percentages of eggs fertilized. 



An investigation of which the results depend on a comparison of 

 the ratio between two classes under different conditions involves, of 

 course, an accurate estimation of this ratio. In our case it is not 

 sufficient to give a rough approximation of the proportion of fertilized 

 to unfertilized eggs as estimated by the eye. The whole investigation 

 is an exact one, and accuracy is as necessaiy in observing the results as 

 in carrying out the technique of the experiments. 



The percentage of fertilized eggs in a given sample was ;Uways 

 calculated from counts made when the eggs were in the 4-cell stage. 

 This stage is reached at a convenient time after fertilization, and it is 

 impossible to mistake a segmenting for an unsegmented egg when the 

 former has divided into four cells. A very necessary preliminar}^ to 

 the operation of couriting is a thorough mixing of the eggs in each 

 dish. It is frequently found that eggs lying in one part of a dish show 

 a slightly different proportion of fertilized to unfertilized from those in 



