Ill 



DIFFERENTIATION 



101 



in the parental types the extremity of the apical arm of 

 Echinus, for instance, is not always club-shaped and that 

 in the hybrid cultures there are to be found, together with 

 variable larvae of more or less intermediate type, individuals 

 with purely paternal characters, though the pure Sphaerechi- 

 nus type never occurs. This contention has been upheld 

 by Morgan and by Steinbruck, the latter of whom has used 

 Strongylocentrotus, the pluteus of which is hardly distinguish- 

 able from that of Echinus, as the male parent. Steinbruck 



FIG. 19. Pluteus of Echinus microtuberculatus from in front and from 



the side, (After Boveri, 1896.) 



finds that the pluteus of Strongylocentrotus is variable (1) in 

 the termination of the apical arm, (2) in the anal arm, which 

 may fork, or be double or even treble throughout, though 

 cross-bars are not seen, and (3) in the length of the oral and 

 of the transverse arms. In the pluteus of Sphaerechinus 

 (1) the apical ' frame ' may be lacking, (2) the apical arms may 

 fork and meet one another, but not be met by the oro-apical 

 branches, and (3) the cross-bars of the anal arm may be few 

 or absent. Most of the hybrids are intermediate, there is 



