SPICULES OF SYNAPTA AND ATTRICULARIA LARVA. 501 



with a single nucleus only measure O'Ol mm. Nevertheless, 

 after the secretion of the calcareous wheels they expand still 

 further ; for if we examine the wheels in alcoholic prepara- 

 tions . . . we can distinguish a distinct 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 expan- 

 sions. 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 state- 

 ments as to the formation of the wheels in the Auricularia 

 reveal a mode of development which at first 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 amcfiboid 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 mem- 

 brane which assumes complex folds, and that in 

 this definitely circumscribed mould the casting 

 of the hard parts ensues," 



I have quoted Chun at length because his remarks show 

 how carefully most of his observations wei'e made and how 

 purely accidental his mistakes were. I will now describe the 

 mode of formation of the wheels and globes in the Auri- 

 cularia larva as it really occurs. I am unable to say for 

 certain as to whether the syncytium which deposits the 

 spicule arises from one cell solely by the simple multiplication 

 of its nucleus as Chun describes, or as to whether it is afso 

 formed in part by the fusion of originally separate scleroblasts. 

 I have certainly seen cells containing two and three nuclei, but 

 I have also seen numerous clusters of separate scleroblasts 

 which, unless they take on some other function, must coalesce 



VOL. 51, PART 3. NEW SERIES. 37 



