MR MINCHIN ON SPONGES. 6 



O 



As a simple type may be taken the development of ClatJirina 

 blanca (= Giiancha blanca Miklucho-Maclay, Ascetta blanca Haeckel). 



The Segmentation is total and regular (Metschnikoff). The 

 Blastula becomes oval, and the blastomeres are transformed into 

 columnar flagellated cells all round, except at one pole (posterior), 

 where there are two large granular cells, which are simply undif- 

 ferentiated blastomeres. 



The Larva is set free and swims for 24 hours ; meanwhile an 

 inner mass arises by modification of some of the cells of the outer 

 layer. Here and there a flagellated cell retracts its flagellum, 

 becomes amoeboid, and passes into the interior. By the end of the 

 larval period the cavity becomes filled up, and the larva in con- 

 sequence shrinks in size. 



The Larva fixes by its anterior end ; the inner mass bursts out, 

 and forms an epithelium which surrounds the flagellated cells ; only 

 a few cells of inner mass remain in the interior, to become the 

 first porocytes. The posterior granular cells break up into a great 

 number of minute cells (ist day). 



Next the porocytes come together in the centre and a cavity 

 appears between them, the gastral cavity, from which the flagellated 

 cells are at first excluded. At the same time cells of the dermal 

 epithelium immigrate to form the skeletogenous layer (2nd day). 



The gastral cavity enlarges, so that the porocytes become 

 separated one from another, and the cavity becomes bordered by 

 the former ciliated cells, which now acquire collars and flagella. 

 The porocytes migrate outwards, and form the pores. The osculum 

 is formed by a breaking through of the body wall (3rd — 5th day). 

 The minute cells which arose from the breaking up of the posterior 

 granular cells of the larva become the wandering cells, amoe- 

 bocytes. 



Note in the above development, 



(i) The first differentiation is into ciliated cells and amce- 

 bocytes. 



(2) The second differentiation is brought about by a modifica- 

 tion of some of the ciliated cells to form an inner mass, the future 

 dermal layer ; the unmodified ciliated cells give rise to the gastral 

 la)'er. 



(3) In the Metamorphosis, the reversal of position of the larval 

 layers. 



(4) The gastral cavity is at first surrounded by the porocytes, 

 just as in the adult when a completely contracted sponge is 

 becoming expanded. 



(5) There is no true mesoderm, but only a functional differen- 

 tiation of the Dermal Layer into an epithelial (contractile) Ia}'er 

 and a skeletogenous parenchyma. 



(6) The origin of the amcebocytes, and hence of the sexual 

 cells from the posterior granular cells, in other words, from simple 

 blastomeres. 



