402 DAVID H. TENNENT 
We are at a point now where we may gather together the many 
different accounts that have been given and restore order in a 
field which has been in a state of disorder. We have conclusive 
evidence that under certain conditions, most of the embryos 
of a given cross will have a skeleton of a definite type and we also 
know that a smaller number may depart radically from this type. 
Thus we may obtain from eggs of one female fertilized by sperm 
from one male a ‘complete series of skeletons ranging from the 
purely maternal to the purely paternal form. 
When we compare these series numerically with a si* ular series 
of zygotes from some species, in the cleavage and slightly later 
stages we find the same sort of variation in the number of retained 
maternal and paternal chromosomes. In other words, there is 
as great a variation, of its kind, in the kind of chromosomes in 
cross fertilized eggs as there is in the kind of skeleton in hybrid 
plutei. This is not true for all species. In some hybrids all of 
the chromosomes are retained and a dominance of one kind of 
skeleton over another is exhibited. 
The facts regarding the retention or elimination of chromosomes 
and the character of the ensuing pluteus are of interest. Pre- 
sented briefly they are: 
1. Elimination of no chromosomes and dominance of one species 
over the other with respect to the character of the skeleton. 
Examples: 'Toxopneustes ¢ X Hipponoé ¢ 
Eechinus ¢ xX Antedon ¢ (Baltzer) 
Strongylocentrotus ¢ x Antedon ¢ (Baltzer) 
2. Elimination of part of the chromosomes and dominance of 
one species over the other with respect to the character of the 
skeleton. 
Examples: Hipponoé ¢ XxX Toxopneustes 2 
Echinus ¢ X Sphaerechinus ~ (Baltzer) 
Strongylocentrotus ¢ Xx Sphaerechinus < (Baltzer) 
3. Elimination of no chromosomes and skeleton of intermediate 
character. 
Examples: Sphaerechinus ¢ X Strongylocentrotus ~ (Baltzer) 
Sphaerechinus ¢ X Arbacia ~ (Baltzer) - 
