38 ' Hjalmar Théel, 



solidified in the shape of reserve granules while the rest remains in 

 fluid form. When the formation of a deposit is going to begin, the 

 calciferous cells combine by means of their clear pseudopodial peripheral 

 plasm protruding and flowing together, thus constituting a common 

 clump of clear living plasm outside the granular main portions of the 

 cells and between these and the blastula-wall. This pseudopodial clump 

 forms the organic ground-substance, in which the calcification takes place, 

 calcareous salts in fluid form being continuously carried to it and there 

 converted into solid form. During the act of solidifying in this organic 

 ground substance, the resulting calcareous body assumes by degrees the 

 shape of a tetrahedron, which gradually increases in size by concentric 

 deposition of salts, layer upon layer, and changes into a small three- 

 armed star. 



The further increase of this early calcareous body is efi"ected by 

 the activity of the same calciferous cell, the granular main portion of 

 which may be able to change its place along the arms of the out- 

 growing spicule, as well as by new cells arriving and entering into 

 communication with the original common pseudopodial plasm, which 

 still remains in a living state round the deposit as a thin membrane-like 

 investment. The junction between the primitive calciferous cells and 

 the additional ones takes place of course in the same way as above 

 mentioned, by their peripheral clear plasm coalescing and getting mixed 

 with the membrane-like plasm investing the increasing deposit, the re- 

 sult being that all the calciferous cells cooperate in forming the organic 

 ground-substance for the outgrowing spicule and in providing this sub- 

 stance with new calcareous salts in liquid form, which subsequently be- 

 come deposited round the spicule. i 



Thus we see that the calcifying matrix, originated from the ecto- 

 plasm of many calciferous cells, predetermines the form of the increa- 

 sing deposit and gives rise to the organic substance in the deposit as 

 well as to the thin coat or membrane which investis it. Besides, the 

 cells in question need not be closely packed, but lie sparsely, and in 

 full-sized spicules they become rare in such a degree, that one is 

 tempted to suppose that they have parted from the calcifying matrix in 

 order to constitute subsequently new centres of calcification. 



As a rule, calciferous cells are present at such parts of the in- 

 creasing spicule, where branches protrude, apparently caused by them, 

 but this seems not always to be the case in the formation of unimpor- 

 tant spines. This fact cannot be explained otherwise, than that the 



