the Skeletal Parts in Echinoderms. 235 



In rare instances they advance as far as the middle of the 

 spokes or even to the periphery. 



No secondary multiplication of the spokes of the wheel 

 takes place ; their number corresponds exactly with that ot 

 the undulating evaginations of the newly formed internal 

 membrane, which develop into radiating tubes. As is well 

 known, the number of the spokes varies ; in the case of the 

 Auricularia from the Canaries we find from thirteen to 

 eighteen. 



Since the diameter of the fully formed calcareous wheels is 

 found to be from "09 to '1 millim., it follows that a tenfold 

 enlargement of the diameter of the skeletogenous cells takes 

 place, since the latter in the stage with a single nucleus only 

 measure '01 millim. Nevertheless after the secretion of the 

 calcareous wheels they expand still further ; for if we examine 

 the wheels in alcohol preparations (the delicate points referred 

 to can scarcely be demonstrated in glycerine and Canada 

 balsam), we can distinguish a distant periphery formed by a 

 delicate membrane, from which, alternating with the spokes 

 and almost equalling them in length, membranous tubes 

 arranged in the shape of a star run to the periphery of the 

 wheel, where they usually exhibit flask-shaped expansions. 



On careful decalcification of the wheels by means of weak 

 chromic acid it is easy to show the nuclei and the contour of 

 the wheel in the shape of a delicate membranous envelope 

 within the skeletogenous cell. 



The above statements as to the formation of the wheels in 

 the Auricularia reveal a mode of development which at 

 present appears to be unique. While the skeletal pieces of 

 Echinoderms were hitherto essentially regarded as intercellular 

 structures, the formation of which was due to several mobile 

 amoeboid cells (I am well aware that more recent observers 

 are inclined to attribute the shape of the skeletal elements 

 without hesitation to directly mechanical influences), we now 

 find that the form of the calcareous wheel is traced out within 

 a multinucleate cell by means of an organic membrane which 

 assumes complex folds, and that in this definitely circumscribed 

 mould the casting of the hard parts ensues. 



