662 
DAVID H. TENNENT 
Of these characters I believe that only observations on the skel- 
eton are of practical value. The skeleton in the larvae of each 
species is of perfectly definite, i.e., a specific form. The species 
selected for crossing should be those whose plutei have character- 
istic skeletons. 
The chromatophores are amoeboid in nature. I have frequently 
seen them move out upon the surface of the body and off into the 
surrounding medium. The arrangement is not specific. The 
pigment content, although different in various forms, is an ex- 
ceedingly difficult character to handle, the shades in color from 
dark red, almost a brown, into bright red, rendering the use of a 
color chart imperative. 
The number of primarj^ mesenchyme cells is a character which 
may possibly be of value in a study of blastulae, but cannot be 
made use of in the examination of older larvae. 
The size of the larvae is extremely variable even in eggs from 
the same individual. 
The skeleton, on the other hand, is a structure which offers 
many advantages as a basis for comparisons. It may be readily 
seen and measured, the normal variations in a given species may 
be determined, and it is a structure which shows at once the influ- 
ence of a cross if the cross has been made between properly se- 
lected species. 
The material upon which this paper is based consists of twenty- 
four series of eggs, nine of which were studied in detail and mea- 
sured. 
The paper may be conveniently divided into three sections. 
Section 1. A statistical consideration of variation, 
Section 2. Potential egg variations. 
Section 3. Discussion. 
1. A STATISTICAL CONSIDERATION OF VARIATION IN ECHINOID 
PLUTEI 
In this section nine series of embryos will be considered, each 
series derived from the eggs of a different female fertilized by 
sperm from a different male. We are dealing then with the 
