STUDIES ON THE DEVELOPMENT OF ECHINOIDI5A. 301 
chyme cells, (c) general form. But the first thorough investi- 
gation of the characters of the larvse of the genera Echinus 
strongylocentrotus and Sphmrechinus and of the hybrids 
produced by crossing these species was made by Vernon 
( 23 , 24 ). It must be borne in mind that it is possible to 
fertilise the eggs of Sphaerechinus with the sperm of 
Echinus and Strongylocentrotus, but that eggs of 
Echinus or Strongylocentrotus ^ generally refuse to develop 
when fertilised by the sperm of Sphaerechinus. Vernon 
came to the g’eneral conclusion that the predominance of 
paternal or maternal characteristics in the hybrid offspring 
was a question of the relative sexual ripeness of the male and 
female parents. 
It may be incidentally remarked that thelarvmof Echinus 
and of Strongylocentrotus are not separable from one 
another by any clearly defined characters, but that those of 
Sphaerechinus are sharply separated from the others by 
having the skeleton supporting each of the two post-oral 
arms in the form of a lattice-work consisting of several 
parallel calcareous rods joined to one another by numerous 
cross-bars. The skeleton of the post-oral arms of the other 
genera consists simply of single bars. It follows that the 
main feature relied on in determining which parent’s influence 
predominates in the hybrid is the character of the skeleton 
of these arms. 
Vernon’s work was criticised in 1902 in a remarkably good 
paper by Steinbriick ( 20 ), in which he dealt with the cross 
between Strongylocentrotus and Sphaerechinus. Stein- 
briick called attention to the fact that in their so-called 
distinctive characters the larvae of both forms are variable; 
that in pure cultures of Strongylocentrotus larvae are 
occasionally met with which possess two calcareous rods with 
occasional junctions between them in the post-oral arms, and 
that in pure cultures of Sphmrechinu s, larvae are some- 
times found in which the lattice-work is partly abolished in 
‘ Driesoli (5) records that 1 per cent. of Strongylocentrotus eggs 
develop when fertilised with Sphaerechinus sperm. 
