E. MAESHALLI. — DEVELOPMENT OF HEXASTEES. l\)ô 



teristic axial filaments in the principals (see p. 51). It may 

 therefore be said that a hexaster begins its àevelopnent as a 

 hexactin. The terminals are appendages which are later added to 

 the principals (see p. 56), 



It is highly unsatisfactory that earlier stages than the one 

 just mentioned — stages which might have shown the mode of the 

 first formation of hexaster principals and therefore of hexactin 

 rays — have not been discovered. I should think it not unlikely 

 that these might be found to originate each as a separate sclerite, 

 as was so beautifully shown by Minchin ('98) to be the case 

 with the rays of triradiates and quadriradiates in the Calcarea.''' 



The hexradiate principals, during the entire period of develop- 

 ment of both the floricome and the graphiocome, are imbedded 

 in a body of protoplasmic substance inclosing a crowded number 

 of nuclei. This nucleated substance may not improperly be called 

 the sclerohlast-mass, for reasons which I think are obvious. At 

 first, so long as the terminals are yet undeveloped or are very 

 short, the mass may be said to present a more or less octahed- 

 ral shape, with somewhat concave surfaces and with rounded 

 corners (figs. 29, 30, 32). In it the three axes of the principals 

 are disposed similarly to the axes in a crystal octahedron, the 

 outer ends of the principals coming up very close, but I think 

 normally not quite, to the surface at the six rounded corners. 

 The mass may otherwise be described as having its surface raised 

 into six, radially directed, hump-like protuberances by the six 

 principals contained within. Later, after the terminals have 



* I may here once more call attention to the facts, mentioned before, viz., that the 

 first spicules which arise in the larvte of Leueopsacus orihodocus and Vitrollula fertile are 

 stauracliuK, and that these are formed in the periphery of tlie inner cell-mass in a very early 

 larval stage which shoivs as yet no indication vliatever of the chambers, the flagellated cells still 

 forming a covering layer on the external surface. These facts may contain hints of great 

 significance as to the phylogeny of spicules in the Hexactinellida. 



