EARLY DEVELOPMENT OF GRANTIA COMPRESSA. 287 



lie describes the formation of the " Spermatocyste " by means of cell which 

 approaches the spermatogonium ("Spermazelle"^, and surrounds it com- 

 plete^'; he figures and describes the sperm-cell dividing inside the cyst-cell. 

 If Gorich's account of the formation of the nutrient-cell capsule is correct, 

 it would mean that in Grantia probably all the cells in PI. 19. fig. 5, marked 

 NSC, would have originated from a single nurse-cell. I doubt this, but 

 am unprepared at present either finally to reject or adopt Gorich's account. 

 In Gorich's spermatid and spermatozoon there is no nucleolus. 



(g) The Collar-cells as Ihe Dominant and most Characteristic Tissue 



of the Sponge. 



Dendy (2) remarks, " The one constant and characteristic feature about 

 sponge histology is, of course, the collared cell, and that is only constant in 

 the sense that its typical form is that which possesses a collar and a flagellum. 

 The sponge is, after all, not very much more highly advanced in organisation 

 than a colony of ehoano-flagellute Protozoa." This paragraph aptly describes 

 the views, at which I have myself arrived by my independent observation. 

 The collar-cell is the dominant cell in the sponge, and I feel that Haeckel 

 was correct in tracing the origination of germ-cells from collar-cells. 

 Collar-cells are continually migrating into, and reinforcing, the mesoglea 

 of the growing sponge ; the collar-cell is really very little differentiated 

 except for the flagellum and collar, which, we are now quite sure, can readily 

 be withdrawn. The collar-cell then becomes an active amoeboid cell- 

 individual, whose subsequent fate possibly may be to metamorphose into a 

 germ-cell. 



One is obliged to remark that in the discussion on the origin of germ- 

 cells — not only in sponges, but in other animals — there has been far too much 

 reliance set upon theoretical conceptions, and not enough on simple observa- 

 tion. It does not suffice for critics to state that they "' do not think that 

 collar-cells can metamorphose into germ-cells'" ; they must bring forward 

 something more definite before one can give their views adequate attention. 

 So far as I am able to understand, the critics of Haeckel 's view as to 

 the origin of sponge germ-cells base their assumptions on the illogical 

 and unnecessary extension of Weismann's views to sponge embryology. 

 According to these observers, germ-cells are supposed to be derived from 

 non-differentiated embryonic elements, and the collar-cell, having become 

 differentiated, is debarred from entering the charmed circle of cells which 

 alone can become germ-cells. 



Speaking for myself, I am quite unable to see why a collar-cell cannot 

 differentiate, especially when one remembers that an embryonic cell can 

 become a collar-cell. If the stimulus can be provided to make a cell undergo 

 certain changes in one direction, why should not other stimuli cause the 

 changed cell to pass back along its former path ? The collar-cell is not a 

 highly differentiated cell, such as a metazoan ganglion or gland-cell. 



