STUDIES IN SPICULE FORMATION. 299 
Now, according to the second of the two above stated 
principles, further growth of the spicule substance, i.e. 
further protrusion of the protoplasmic investment into the 
surrounding medium, will take place at that portion of the 
spicule surface possessing the greatest angularity or rather 
Text-Fic. B. 
convexity, 1.e. at the rim of the amphiccelous extremity, or in 
other words, at any or all of the four “corners” always 
visible when the spicule is viewed from a side aspect. If the 
external medium were absolutely homogeneous in constitution, 
Text-Fic. C. 
extension of the spicule substance would, under such con- 
ditions, take place uniformly at the rim of the spicule ex- 
tremity, and a hollow cone-shaped structure would be 
produced, but in Alcyonium digitatum and the vast 
majority of other Alcyonaria, the surrounding mesoglcea is 
not by any means homogeneous in nature, and hence the 
