324 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



The cell division does not take place in the same manner 

 throughout. In the Calcispongire the young Morula consist of a 

 ring of cells with a hole right throiigh the centre, connecting the 

 oral pole of the larva? with the aboral. In other Sponges such a 

 stage does not exist, but only a solid morula which produces 

 secundary in the interior, a space, or a Furchungshohle. All 

 Sponges seem, however, to pass a Blastula stage. The single layer 

 of cells which constitutes the wall of the Blastula may be produced 

 by the coalescing of the oral and aboral apertures (Calcispongiae), 

 or by the ordinary process of an imbibition of water by the solid 

 morula. 



In the central mass, which Marshall calls Ccenoblostem, first 

 Nuclei seem to make their appearance. Marshall thinks that the 

 whole Ccenoblostem resembles a Syncitium. Other observers are 

 inclined to consider these "Nuclei as belonging to cells, the limits 

 of which escape observation. 



In this state the embryo leaves the brooding place, swims about 

 for some time in the canals of the mother and finally leaves them. 

 The cells of the outer layer, which now can be called Ectoderm, 

 produce one cilia each, and the movements of these cilia propel the 

 young Sponge. In some groups (Ceraospongiae, Reniera, and other 

 Silicispongiee) the embryo has a pigmented spot on one end of the 

 oval body, which might perhaps be considered as an eye. The 

 embryos of Reniera filigrana, a Sponge which grows on the lower- 

 side of stones, &c, shuns the light and always seeks the darkest 

 corner of the aquarium. In some cases a ring of particularly 

 long cilia surround the pigmented spot. The bulk of the Ccenoblo- 

 stem increases in size continually and soon bursts the formerly 

 continuous Ectoderm and protrudes slightly from the apertures. 

 The Ectoderm is always torn at the end of the rotation-axis of 

 the oval Embi'yo. The pigmented cells are 'hereby pushed aside 

 and form a ring which surrounds the naked Ccenoblastem. The 

 Ectodermal cells retract their cilia and the embryo, after showing 

 irregular movements for some time, finally affixes itself. The 

 Ccenoblastem flows towards the aboral aperture, which is greatly 

 dilitated and soon sticks to the surface, which the embryo has 

 chosen as his abode for life. 



