STUDIES IN SPICULE FORMATION. 41 



shaped spicule as a tetrahedron, and, curiously enough, 

 represents this tetrahedron as forming the distinct centre 

 or basis of the older spicules, secondary calcareous matter, so 

 to speak, prolonging the solid angles of the tetrahedron. As 

 I have stated in Study III, I believe this tetrahedron struc- 

 ture to be quite imaginary, and I certainly cannot credit 

 without more evidence its persistence as the visible basis of 

 older spicules. 



Fewkes (2), describing the metamorphosis of Echin- 

 arachnius parma, says that '^ the first limestone formation 

 which was observed is a trifid spicule in the wall of the body 

 of the growing sea-urchin. In its very first form this trifid 

 spicule is spherical in contour. Later it assumes a trifid 

 shape, and seems to be enclosed in a transparent sac, the 

 outer wall of which is believed to be formed of epiblast, the 

 calcareous body being formed possibly in mesoblast " ! 

 Fewkes also adds that he does not know whether these trifid 

 bodies develop into the plates or not. 



Theel, in his papers on Echinus mi liar is (12) and 

 Echinocyamus pusillus (11), quite correctly describes 

 the development of the spicules, and also states his opinion 

 that " they first originate from cells which have wandered in 

 between the tissues," but he gives no details of the sclero- 

 blastic development. 



Ludwig, Fewkes, and others provide numerous figures 

 showing the young triradiate spicules and older stages, but 

 from not employing suitable staining reagents they entirely 

 overlooked the scleroblasts in connection with the young 

 spicules. 



