472 E. A. MINCHTN. 



am not in a position as yet to give a detailed account either 

 of the monaxon spicules in Clathrina, or of the three- and 

 four-rayed spicules in Leucosolenia; on the other hand, 

 many minute details in the development of these spicules, such 

 as their exact relations to the secreting cells, and so forth — all 

 such details, in fact, as require thin sections for their elucida- 

 tion — remain for the present to be studied. But thin sections 

 of the developing spicules and their formative cells are, as a 

 matter of fact, very difficult to obtain. In the adult sponge 

 new spicules are formed amongst and between older ones, 

 which are often, and indeed usually, present in such masses 

 as to make it hopeless to obtain thin sections without previous 

 decalcification, which, by destroying the spicules, defeats the 

 purpose in view, while thick sections are scarcely more in- 

 structive for the minuter points than are surface views. In 

 the embryo, however, where stages can be obtained con- 

 taining only minute spicules, thin sections of material not 

 decalcified can be made without much difiiculty. I have 

 preferred, therefore, to let the questions of minute detail 

 stand over for the present until I can return to them in the 

 course of my embryological studies, and rather to bring forward 

 my observations as they are than to wait until I can make them 

 exhaustive. In the present memoir I propose to deal with the 

 formation of the equiangular triradiates and quadriradiates 

 rather from the morphological than from the cytological point 

 of view, — that is to say, the number and arrangement of the 

 cells which build them up, and their origin within the sponge. 

 These are questions which for the most part can be studied 

 satisfactorily only in surface views. 



Before, however, proceeding to describe the results of my 

 observations, I feel it my pleasant duty to say a few words of 

 thanks and acknowledgment to those who have assisted me in 

 my work and made it a possibility for me. In the first place 

 I must acknowledge my indebtedness to two ^' pious founders,^^ 

 one of whom has been dead more than six hundred years, 

 while the other, I am glad to say, is alive and well. But for 

 my election to a fellowship on the Foundation of Walter de 



