No. 3.] NOTES ON DEVELOPMENT OF SOME SPONGES. 5 1 3 



solid gemmule splits up into irregular masses of cells. These 

 continue to split up into smaller and smaller masses, the gem- 

 mule meanwhile increasing in size, owing to the absorption of 

 fluid, so that the several masses of cells are distinctly separated 

 from one another (Fig. 2). The splitting up continues until 

 the solid gemmule has been plainly resolved into its constituent 

 cells. The outer cells of the gemmule, very early in this pro- 

 cess of "segmentation," arrange themselves so as to form a con- 

 tinuous layer of flat cells round the periphery. This layer, for 

 convenience' sake, may be spoken of as the ectoderm. Inside 

 the ectoderm at the close of " segmentation " is found a mass of 

 amoeboid cells connected together by their processes and sep- 



FlG. 2. Gemmule of Esperella, undergoing "segmentation." /! = follicle; fs- 

 strands of tissue, by which follicle is connected with canal wall. 



arated by fluid. The flat ectoderm cells next become long 

 slender columnar cells, having pigment in their outer ends and 

 bearing cilia. The metamorphosis of the ectoderm cells does 

 not, however, take place over one pole. Over this pole the 

 ectoderm cells remain flat and without cilia. Further, the inner 

 mass of cells at this pole become steadily denser, until this 

 region of the embryo is occupied by a mass of irregularly polyg- 

 onal cells closely appressed. In the mass of polygonal cells a 

 bundle of long spicules lying in the direction of the main axis 

 of the embryo is developed. In the remainder of the inner 

 contents of the embryo, the cells are less closely packed and 

 are of varioug shapes. The unciliated pole is made the more 



