100 



DIFFERENTIATION 



III 



the male parent (Echinus) from a fertilized enucleate frag- 

 ment of a Sphaerechinus egg, that brief mention may be made 

 of the more recent researches. 



It will be recalled that the plutei of the two genera Echinus 

 and Sphaerechinus differ typically from one another, according 

 to Boveri. The latter (Fig. 18) is short and squat, the oral 

 lappet not divided into lobes, and the skeleton provided with 

 a fenestrated anal arm produced by three long parallel bars 

 united by numerous cross-bars an apical branch to the oral 

 arm and, at the apex, a square 'frame' formed by the union of 

 twigs from the last-mentioned and from the apical arms. The 

 former (Fig. 19) is long and lank, the oral lappet is deeply 



FIG. 18. Pluteus of Sphaerechinus granularis from in front and from 



the side. (After Boveri, 1896.) 



cleft, and in the skeleton the anal arm is not fenestrated ; 

 there is no apical branch to the oral arm, and the extremity 

 of the apical arm is thickened and club-shaped. 



The hybrid larvae (Fig. 20) were described by Boveri as 

 being intermediate in form, shorter and broader than those 

 of Echinus, longer and narrower than those of Sphaerechinus, 

 and having the oral lappet slightly divided; while in the 

 skeleton, the extremity of the apical arm was swollen and 

 branched, the anal arms double but not fenestrated, and the 

 oral arms sometimes provided with an apical branch. 



Seeliger, however, pointed out that there is much variability 



