102 



DIFFERENTIATION 



III 



usually an emargination between the oral arms, and the apex 

 of the body is prismatic. In the skeleton the extremities of the 

 apical arms are simple or branched and antler-like, but never 

 club-shaped ; they may be wide apart, touching, or even fused ; 

 the anal arm may be single, double, or treble, cross-pieces are 

 rare ; the oro-apical branch is usually lacking, and seldom 

 reaches the apical, and the transverse arms generally cross. 



While, therefore, it is possible to say of a whole culture 

 whether it is of the Strongylocentrotus, of the Sphaerechinus, 

 or of the hybrid type, no such assertion can be made of an 

 individual larva. 



FIG. 20. Pluteus of the cross Sphaerechinus granularis ? x Echinus 

 microtuberculatus a*, from in front and from the side. (After Boveri, 

 1896.) 



Under these circumstances the difficulty of determining 

 whether a given character is transmissible from the male 

 parent is obvious. Vernon, however, using Strongylocentrotus 

 for the male and Sphaerechinus for the female parent, found 

 it possible to obtain a culture of purely paternal type in 

 respect of the skeleton, in the winter, when the sexual 

 maturity of the male parent is at a maximum. This is stated 

 by Doncaster to be due merely to the lower temperature, and 

 the same conclusion is reached by Herbst, who has examined 

 the effect of temperature changes on the larvae of the pure 

 parental forms and the hybrid larvae simultaneously. 



The temperatures used were from 11- 19 and from 24-27. 

 Thus in the hybrids the cross-bars of the anal arms are more 



