H. M. FucHS 261 



suspensions after either the eggs or the sperm have been subjected to 

 certain preliminary treatments. Thus the validity of the results depends 

 altogether on the way in which the experiments were carried out ; that 

 is to say, on whether the conditions of experiment allowed of any such 

 numerical comparison of the after-effects and of deductions to be drawn 

 from them. For a detailed description of the precautions taken to 

 ensure that the experiments should be of an exact nature the reader is 

 referred to the Part on " The conditions of self-fertilization in Giona 

 intestinalis " since the methods used were identical with those employed 

 in this case. A short resum^ of the technique is, however, given in the 

 following section. It is thought that these methods will break a new 

 road for the investigation of fertilization phenomena, for work hitherto 

 done on this subject, in which the results have been based on numerical 

 counts, seems to have been carried out under such comparatively inexact 

 conditions that it is usually extremely difficult to judge the extent to 

 which the results are valid. 



II. Methods. 



The method of experimentation was the same as that employed in 

 investigating the conditions of self-fertilization in Ciona intestinalis. For 

 a detailed account of these methods reference should be made to Part I. 



Essentially the method consisted in an exact comparison of the per- 

 centages of eggs fertilized by sperm-suspensions of identical concen- 

 trations, after either the eggs or the sperm had been treated in 

 different ways prior to the fertilization. The results of such treatment 

 were judged by the relative ease or difficulty with which fertilization 

 could subsequently be brought about. The sperm-suspensions used 

 were always so dilute that they would not cause the fertilization of all of 

 the eggs to which they were added. In this way comparisons could be 

 instituted, since the percentages of eggs fertilized in the different lots 

 lay between and 100. 



The term " fertilizing power " of a sperm-suspension is employed in 

 the following sense. If a sperm-suspension of a certain concentration 

 can bring about the fertilization of say 40°/„ of the eggs after it has been 

 treated in a definite way, whereas another suspension of the same 

 sperm, of identical concentration but not having been subjected to the 

 treatment, only causes 20°/^ of the eggs to be fertilized, it is said that 

 the "fertilizing power" of the sperm has been increased by this 

 treatment. 



