164 I. IJIMA : HEXACTINELLIDA. I. 



respects, from the larva by the same general course of develop- 

 ment known in other sponges. 



While it must be admitted that there exists a general agree- 

 ment between the Hexactinellida and other sponges in the 

 histological elements which should differentiate themselves from 

 the internal cell-mass of the larva, the apparent non-development 

 in the former of the connective-tissue matrix but especially of 

 the pinacocytes — admitting it to be the fact — offers a very grave 

 matter for our consideration. Are the naked syncytial trabeculse 

 to be considered as the result of degeneration or as representing 

 the primitive condition of the sponge connective-tissue ? Against 

 either Avay of thinking, the very high degree of organization 

 manifested in the spiculation may appear, from a general point 

 of view, to stand more or less opposed. Nevertheless, I hold it 

 possible that the spicules take their own course of development 

 and complication irrespective of the differentiation of the pinaco- 

 cytes or of the true connective-tissue. At any rate, their formation 

 is known in many sponges (the Hexactinellida included) to begin 

 at a very early larval period, when the internal cell- mass is still 

 in a quite indefinite state of histological differentiation ; and I 

 am inclined to see in the naked syncytial trabeculœ of the 

 Hexactinellida simply a representative of the primitive mother- 

 tissue, from which both the connective-tissue cells and the pina- 

 cocytes of other sponges have later differentiated themselves, — 

 a structure, which may be said to represent both these kinds of 

 cells at once. In favor of this view seems to stand in a measure 

 the fact that the results of modern embryological researches tend 

 to show the origin of the flattened epithelium from the same 

 category of larval cells as the connective-tissue (Maas, Min- 

 CHiN, Evans). 



