ECHINOCYAMUS PUSILLUS. 37 



said I could never observe that the tetrahedron becomes visible before 

 at least three calciferous cells have arranged themselves in a heap 

 close to the blastoderm. But after this is done, the formation of the 

 tetrahedron takes place in a clear pseudopodial plasm situated between 

 these cells and the ectoderm and evidentlj^ derived not from one cell 

 but from all the three cells, the pseudopodia of which have united into 

 a small clump. This clump is made discernible by staining the Bla- 

 stula with some suitable colouring fluid, while the mucilaginous sub- 

 stance which fills up the blastocœl remains colourless, PL I V, fig. 81. 



Thus, according to m.y opinion, the calcareous tetrahedron is a 

 result of the activity of several cells, which deposit calcareous salts in 

 a liquid state in the common pseudopodial clump, where the formation 

 of the tetrahedron afterwards takes place. On account of the transpa- 

 rency of pseudopodial plasm and the opacity of the granular main por- 

 tion of the cells, one gets the impression that the tetrahedron is extra- 

 cellular in position. 



According to Selenka and Semon the tetrahedron not only has 

 originated in a single cell but also arises directly from the calcareous 

 granule of uncertain shape which is present in it, and which conse- 

 quently should form the centre in the future calcareous spicule. For 

 certain reasons I do not think this to be the case. Firstly, it may be 

 remembered, that before the formation of the tetrahedron takes place, 

 there are several cells heaped together in Echinocyamus, each with one 

 or more calcareous granules of uncertain shape. Now it seems rather 

 singular that only one of these granules should be transformed into 

 a tetrahedron, while the remaining ones are probably dissolved and 

 by successive depositions give rise to the further increase o£ the calca- 

 reous body. Besides, it is common to all calciferous cells to possess 

 such granules. For my ovsrn part I cannot explain this in any other 

 way, than that the calciferous cells during their lively activity supply 

 themselves with calcareous salts in such a degree, that it becomes im- 

 possible to keep them in a state of solution. A part of the dissolved 

 salts therefore must solidify, thus giving origin to the granules, which 

 properly may be considered as reserve material, which when required 

 may be dissolved again and used for building up by successive concen- 

 tric depositions the tetrahedron as well as the future spicule. 



I may now be allowed to sketch my own opinion of the way in 

 which the process of calcification goes on. The calciferous cells supply 

 themselves with calcareous matter in such abundance that a part gets 



