Ill 



PORIFEEA 



39 



granular cells encounter the resistance of the spicules, and therefore 

 the embryo becomes hat- shaped. A few small cells are found in the 

 segmentation cavity. Dendy (1889) thinks that these cells, which 

 may be termed mesenchyme, have been budded from the flagellated 

 cells, but this is not certain. The granular cells now proliferate 

 rapidly, especially in the centre, and form a thick mass which 

 becomes invaginated into the blastocoele. 



The embryo is now ready 

 for birth. By the activity 

 of its flagella it bores its way 

 into the adjacent flagellated 

 chamber of the mother, and 

 then escapes through the 

 osculum. During this pro- 

 cess the blastocoele seems to 

 absorb water, the invaginated 

 cells are exserted, and thus 

 the free -swimming larvae 

 acquire an oval form; but 



FIG. 14. View of the embryo of Grantia Idbyrinthica 

 in the blastula stage lying in the embryonic 

 chamber of the mother. (After Dendy.) 



col, collared cells lining a maternal flagellated chamber ; 

 enib, embryonic chamber ; gr, granular cells of the embryo ; 

 mes, cells, so-called mesenchyme budded into the blasto- 

 coele ; spic, maternal spicules. 



FIG. 15. View of the embryo of 

 Grantia labyrinthica in a later 

 stage of development than that 

 represented in Fig. 14. (After 

 Dendy.) 



Letters as in Fig. 14. 



the cells forming one half of the wall of this vesicle are granular and 

 rounded, whilst those forming the other half carry long flagella, and 

 possess, in addition, a bright red pigment. The interior is half-filled 

 up with granular cells. Such a larva is termed an amphiblastula, and, 

 as we shall see, this type recurs in all families of sponges (Fig. 17). 



After swimming for a day or two the amphiblastula comes to rest 

 on the surface of a smooth stone, its ciliated half which preceded the 

 other whilst the larva was moving, being directed downwards. 

 Within a few minutes the larva has undergone an entire change of 

 shape. Its anterior end flattens out; the ciliated cells which con- 



