IV. i INITIAL STRUCTURE OF THE GERM 



259 



The larva of the former (Fig. 156,) 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 larva of Echinus, on the other hand, is long and lank, the 

 oral lappet is deeply cleft, and in the skeleton the anal arm is not 

 fenestrated ; there is no apical branch of the oral arm, and the ex- 



FIG. 157. Pluteus of Echinus microtiiberculatus from in front and 

 from the side. (After Boveri, 1896.) 



tremity of the apical arm is thickened and club-shaped (Fig. 157). 

 Boveri first fertilized the ova of Sphaerechinus with the sperm 

 of Echinus, and so produced a hybrid larva whose characters 

 were intermediate between those of the two parents in the 

 following respects in form, being shorter and broader than 

 that of Echinus, longer and narrower than that of Sphaerechimis, 

 in having the oral lappet slightly divided, and in the skeleton, 

 the extremity of the apical arm being swollen and branched, 

 the anal arms being double but not fenestrated, and the oral arms 

 being (sometimes) provided with an apical branch (Fig. 158). 



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