PHYLUM FORI F ERA 85 



the fertilized egg become arranged as an ovoidal blastula (Fig. 

 50 A) one pole of which is rounded and the other flattened, but 

 from this point onward conditions are not in harmony with the 

 usual plan of embryonic development. The cells at the more 

 pointed pole become extremely granular, while those at the op- 

 posite pole become much elongated and each develops a flagellum. 

 The large columnar cells grow down and partly enclose the mass 

 of granular cells. At about this stage, the larva breaks through 

 the embryonic chamber in which it has developed and by way 

 of the osculum uses the flagella to swim freely in the water. 

 Upon leaving the embryonic chamber of the parent, the granular 

 cells which have up to this time been partially invaginated into 

 the blastocoel come to he on the surface of the embryo. The 

 larva is now an ovoid body one pole of which consists of flagel- 

 lated cells and the other of granular cells. In other invertebrate 

 larvae, the ciliated or flagellated cells are the ones which remain 

 on the surface of the body and form the skin. Hence they are 

 called ectoderm cells. But in the sponges the flagellated cells 

 have a different future and later come to lie inside the body. 

 This free-swimming larval stage is called an amphiblastula. 

 Cells which are the forerunners of the mesoderm occur in the 

 cavity of the amphiblastula. After a day or more of free- 

 swimming existence the amphiblastula settles down with the 

 pole bearing the flagellated cells in contact with some object. 

 The flagellated cells become invaginated to form a cavity within 

 the granular cells and from these the collared flagellate cells 

 develop. The system of pores and canals and the osculum later 

 make their appearance, thus laying down the general form and 

 organization of the sponge, though the details of arrangement 

 of the parts take considerable time for growth and development. 



During the period of transformation from the larva some of the 

 mesenchyme cells from the interior of the larva break through the 

 layer of granular cells and become distributed over the surface. 

 These apparently mesodermal cells of the larva thus form the 

 body covering, adding another unparalleled chapter to the history 

 of sponge development, for in no other instance is there a record 

 of the skin of an animal being formed from mesoderm. 



Skeleton. — The skeletal structures (Fig. 51) which give the 

 chief character to the Porifera are spicules formed by mesoderm 

 cells called scleroblasts. Characteristically each of the large 

 spicules of the mature sponge starts as a microscopic crystal 



