MATERIALS FOR A MONOGRAPH OP THE ASCONS. 473 



Merton, it would, perhaps, have been impossible for me to 

 have engaged in these researches at all. To Professor de 

 Lacaze-Duthiers, on the other hand, founder of the marine 

 laboratories of RoscofF and Banyuls, I am under the greatest 

 obligation for his generous hospitality. On the frequent 

 occasions on which I have visited these stations I have been 

 made entirely free of all their resources, not only of laboratory 

 and aquarium, but even of lodging, and have been treated 

 with that uniform kindness and consideration which in France 

 is always shown, without regard to race or nation, to those 

 whom the fame of French scientific institutes attracts as 

 visitors and students. In this way I have had at my disposal 

 a rich and varied fauna, especially at Banyuls, the value of 

 which can scarcely be estimated. 



I must further express my warm thanks to Professor Ray 

 Lankester for his kindness in giving me a place in his labora- 

 tory at Oxford, which has been, so to speak, my permanent 

 head- quarters, where the greater part of my studies have been 

 conducted. I am also indebted to him for much help and kind 

 encouragement in his double capacity, both of Professor in 

 the University and Editor of this Journal. Finally, I have to 

 tender my thanks and gratitude to Professor R. Hertwig, of 

 Munich, for the kindness and hospitality which he extended 

 to me during a three months' stay at Munich last year, making 

 me free of his laboratory and helping me in every way. 



To each and all of those I have mentioned, whether of the 

 thirteenth or nineteenth centuries, I wish to acknowledge my 

 obligations and express ray sincere thanks. 



I. Introductory. 



Before approaching the subject of spicule development a 

 few words must be said, first, as to the technique employed, 

 and secondly, upon the structure and histology of the sponge. 

 lu order to understand properly the origin of the spicule- 

 forming cells, and to distinguish these cells from other tissue 

 elements, it is necessary to be acquainted with the forms and 



