STUDIES IN SPICULE FORMATION. 147 
posits is unknown; in other words, no one has yet found 
siliceous equivalents of the triradiates and quadriradiates of 
Calearea. This being the case, it was with some interest 
that I read an account of this very process which was stated 
to occur in connection with the spicules of Tethya lyncu- 
riim. This account was contained in a paper published by 
Dr. O. Maas in 1900 (24). The exceptional nature of the 
statements in this paper led me to pay special attention to 
the scleroblastic development of the spicules in Tethya 
lyncurium, but, as on a former occasion,' I was unable to 
confirm Dr. Maas’ results. Maas makes the following state- 
1 To enable the reader to estimate the value of Dr. Maas’ reply (8) I will recall 
the facts. Minchin in 1898 (4) showed that each triradiate spicule of the Ascon 
sponges which he investigated originated as three separate rods radiating 
from a common centre, each ray being produced by a pair of cells of the 
‘sextet,’ and that these rods only secondarily became fused together at their 
inner extremities to form the triradiate spicule. In the same year (la) and 
in 1900 (2) Maas published papers, each dealing in part with the similar 
spicules in Sycons. In the latter paper (and with this I am chiefly concerned) 
Maas described a process of spicule formation as occurring in Sycons which is 
radically different from that which occurs in Ascons—each triradiate spicule, 
as a whole, being stated to arise as a single concretion in one mother-cell. At 
Professor Minchin’s suggestion J, during 1903 and 1904, devoted the greater 
part of a year’s work to ascertaining for certain whether the process of spicule 
formation in Sycons was so fundamentally different from that obtaining in 
Ascons, as alleged by Maas, and I found, both to Professor Minchin’s and my 
own satisfaction, that the mode of spicule formation in three species of Sycon 
closely allied to those studied by Maas was identical with that found in 
Ascons. This being the case I considered that Maas’ statements were erro- 
neous, and I ventured to express that opinion—perhaps in too brusque a 
manner. Dr. Maas in his counter-criticisms says that I obtained anomalous 
results owing to the method of fixing the Sycons which I adopted—a criticism 
which applies to Professor Minchin’s work as well as my own. I can only 
reply that Professor Minchin’s slides (and my own to a less extent) have been 
inspected by numerous zoologists, and that no fault has ever been found with 
them on the score of imperfect histological preservation. Maas’ criticism 
simply proves, what indeed he admits, that he has never seen the three-rod 
stage of the triradiate spicule, else he would not assume it to be an artefact. 
Maas again speaks of the initial stages of spicule deposition as if, owing, as he 
states, to their small size and transitoriness, it were almost impossible to 
observe them. This is not the case. They are easily to be observed, pro- 
