232 M. Carl Chun on the Formation of 



siil>ptancc of the lavvfc (it was scarcely likely that Sclcnka 

 and ]k'roiiarcl should still retain the ideas they formerly 

 expressed as to an ectodermal origin of the calcareous bodies 

 of the integument), it was recently sought by Selenka and 

 Semon to determine the finer processes wliich take ])lace in 

 the secretion ol the calcareous matter. According to Semon's 

 account there arises within tlie skeletogenous mesoderm cell a 

 tetrahedron, which subsequently dcveloj)S into the triradiatc 

 body already observed by Selenka and passes out of the cell 

 enveloped in a delicate membrane. Tiie triradiate body is 

 then a])proached by other mesoderm cells, which enlarge it, 

 and by means of complicated furcations transform it into the 

 definitive calcareous structure. Semon's observations have 

 been confirmed by a careful memoir by II. Thcel, which has 

 just appeared (" Development of Echinocyamus jnisillus,^^ 

 E. S. Upsala), in so far as Theel also claims the tetrahedron 

 developing into a triradiate body as the foundation for the 

 building-up of the skeletal parts of the Pluteus. It is true 

 that there are material discrepancies in the observations as to 

 the earliest origin of the tetrahedron. For [according to 

 I'hc^el] it is formed between at least three cells in a clear 

 organic basal substance, and therefore from the outset proves 

 itself to be an intercellular skeletal element, in the enlarge- 

 ment of which a large number of amoeboid cells subsequently 

 take part. 



xsow Semon is inclined to regard the tetrahedron, which 

 develops into a body with three or four rays as the case may 

 be, as the universal starting-point of the whole of the skeletal 

 structures of Echinodcrms. This view, then, would also 

 sanction the converse inference, that the individual calcareous 

 structures represent the product of a larger number of meso- 

 derm cells. Nevertheless this assum])tion does not hold for 

 all skeletal elements, inasmuch as, e. g., the wheels of the 

 Axiricularia and the anchors and anchor- j)lates of the Synap- 

 tida^, according to the statements of older investigators, whicii 

 in essential points were recently confirmed by Semon and 

 Ludwig, do not exhibit a tetrahedron and body with three or 

 four rays as a starting-point. Semon therefore believes that 

 the original condition became obliterated in these instances, 

 and that the appearance of a star with six rays, which he 

 gives as the basis of many calcareous wheels, implies a 

 curtailment of the primitive arrangement. It would lead us 

 too far, should we wish to discuss this conception liere ; 

 against it the objection may always be advanced that the 

 calcareous wheels clearly represent quite primitive structures, 

 which not only furnish the distinctive character of ihii Auricu- 



