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, Seeliger, 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 scason 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. 
Spherechinus gives throughout the year mature eggsand sperm, 
although in summer the percentage of larve that develop is 
smaller. If the eggs of Spherechinus are fertilized by sperm 
of Strongylocentrotus during the summer months, May, June, 
July, the hybrid larve resemble the Spherechinus type (the 
mother), although some of them (4 or less) show traces of the 
paternal (Strongylocentrotus) type of larve. 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 larvee its 
own characters. 
