GROWTH OF IDEAS 273 



591 species. All this confusion was saved by the 

 Darwinian idea of not setting up absolutely rigid 

 classes, families, genera, and species. But even 

 this was not yet the essential point. 



As he had done in the case of the siphonophores, 

 Haeckel endeavoured to derive as much informa- 

 tion as possible from the "ontogeny," or embryonic 

 development, of the calcispongiae. He established 

 in some cases, it seemed to him, that a single 

 calcisponge-individual at first and up to a certain 

 stage developed from the ovum in the same way 

 as a medusa or a coral or an anemone. The fer- 

 tilised ovum, a single cell, divided into two cells, 

 then several, and at last formed a whole cluster of 

 cells. In this cluster the cells arranged themselves 

 at the surface, and left a hollow cavity within. 

 Then two layers of cells were formed, like a double 

 skin, in the wall of this vesicle, and an opening 

 was left at one spot in the wall of it. Thus we 

 got a free-swimming embryo, with a mouth, an 

 external skin, and an internal digestive skin or 

 membrane. Then the creature attaches itself to 

 the floor of the sea and becomes a real sponge, 

 partly by developing along its characteristic lines, 

 and partly (in most cases) by producing other 

 sponges from itself in the form of buds, like the 

 siphonophore, and so forming an elaborate colony, 

 to which we give collectively the title of "a 

 sponge." These facts led to the following re- 

 flections. 



This original development from the ovum, first 

 into an embryo with the form of a small globe or, 



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