48 



INVERTEBRATA 



CHAP. 



which is the centre for the formation of the gemmnle. Here they 

 become massed together so as to form a spherical lump. Other 

 wandering cells with less yolk follow after them and form a layer 



FIG. 25. Longitudinal sections of Amphiblastula larva, just fixed larva, and young 

 sponge of Leucosolenia variabilis. (After Minchin. ) 



A, free-swimming larva. B, just fixed stage. C, young sponge four days old ; cent, central cells ; 

 fl, flagellated cells ; gr, granular cells ; int, intermediate cells : I, lens ; pig, pigmented cells. 



surrounding them, in which the individual cells, as a consequence of 

 mutual pressure, assume a columnar form. The investment is for a 

 considerable time not quite complete, and an opening therefore exists. 

 In this opening are found some specially large cells called tropho- 



