190 PROBLEMS OF FERTILIZATION 



exceedingly small. In several instances the eggs were 

 staled for several hours, even up to twenty-four, before 

 fertilization, with resulting increase in the percentage of 

 eggs fertilized; but in the case of Sphaerechinus ferti- 

 lized by Strongylocentrotus this treatment was not suc- 

 cessful. It is important to note that specificity always 

 appeared with reference to the relative ease of fertiliza- 

 tion with the specific and foreign sperm. 



Tennent (1910) made eleven crosses within the order 

 in which the reciprocals belonged to different genera, 

 families, or suborders; and many cross-fertilizations 

 within the order have been made by other observers. 

 Unfortunately for our purpose these studies have been 

 made from the point of view of heredity or chromo- 

 some behavior, and the fertilization problems have been 

 referred to only incidentally as a general rule. They 

 are, however, sufficient to show that the chances of 

 success of a cross cannot be postulated wholly on the 

 systematic position of the species. Thus Tennent 

 reports that the cross between Moira $ and Toxo- 

 pneustes $ belonging to different suborders takes place 

 very readily and the larvae develop well. The recip- 

 . rocal cross can also be made, but succeeds best if the 

 Toxopneustes eggs are allowed to stand in sea-water 

 five hours before being fertilized. Hipponoe $ crossed 

 with Cidaris 6, also belonging to different suborders, 

 gives poor results; no fertilization membrane is formed, 

 segmentation is irregular, larvae abnormal. 



Reciprocal crosses are sometimes quite similar with 

 reference to fertilization; but frequently they are not. 

 Thus Fischel (1906) reports for the crosses between 

 Strongylocentrotus and Arbacia that fertilization never 



