PORIFERA 



15 



tube with a flattened basal surface; and since they do not 

 come into close contact with one another at the centre, they 

 enclose between them a cavity (cleavage cavity) open above 

 and below. With the next act of cleavage each of these four 

 cells is divided into two equal parts by a new meridional 

 furrow (Fig. 1 G and D) . The embryo now consists of a circle 



Pig. 2.— Cleavage stages of Sycandra raphanus (after F. E. Schulze). A, two- 

 cell stage ; B, four-cell stage ; C eight-cell stage ; D, the same in vertical section in 

 its relation to the collared epithelium of the maternal radial tube (diagram) ; E, 

 sixteen-cell stage ; F, the same in vertical section (diagram) ; G later stage of 

 cleavage with eight granular (ectodermal) cells at the lower pole; H, blastosphere 

 stage in side view. In the interior the cleavage cavity ; granular cells below, 

 otherwise an epithelium of tall columnar cells. 



