Behavior of the Germ-cells 175 



most cases this does not hold. Eggs lose, as a rule, rather than 

 gain in their responsiveness to foreign sperm if kept too long. 



Experiments with Echinoderms 



A number of investigators have made crosses between different 

 species of sea urchins with varying success. In recent years 

 Vernon, Boveri, SeeHger, Morgan, Driesch, Herbst; Loeb, and 

 Godlewski have carried out experiments with these forms. 

 Vernon found that out of sixty- four possible combinations, 

 forty-nine gave the following results : twenty-nine developed 

 to the pluteus stage, nine to the segmentation, blastula, or 

 gastrula stages, and in eleven fertilization did not take place. 



Vernon tried to show that the characters of the hybrid embryo 

 are dependent upon the relative ripeness of the eggs and of the 

 sperm in the two species that are crossed. He carried out his 

 experiments with several species of sea urchins found in the 

 Mediterranean. The breeding period of these animals extends 

 over several months, or even in one species throughout most of 

 the year. The height of the breeding season may be different 

 for different species. During the time preceding and following 

 that of the full maturity of the eggs and sperm, the eggs may still 

 be fertilized, although fewer of them develop normally. For 

 example: — the eggs of Strongylocentrotus reach their optimum 

 in December or January, their minimum in July or August. 

 Sphaerechinus gives throughout the year mature eggs and sperm, 

 although in summer the percentage of larvae that develop is 

 smaller. If the eggs of Sphaerechinus are fertilized by sperm 

 of Strongylocentrotus during the summer months, May, June, 

 July, the hybrid larvae resemble the Sphaerechinus type (the 

 mother), although some of them (^ or less) show traces of the 

 paternal (Strongylocentrotus) type of larvae. In November 

 the hybrids approach more nearly the type of Strongylocen- 

 trotus (the father), and in December are entirely of this pater- 

 nal type. In other words, as the sperm of Strongylocentrotus 

 becomes more and more mature, it transmits to the larvae its 

 own characters. 



