98 JOURNAL OF THE 



from one another. The splitting up continues until 

 the solid gemmule has been plainly resolved into its 

 constituent cells. The outer cells of the genimule, very 

 early in this process of "segmentation," arrange them- 

 selves so as to form a continuous layer of flat cells round 

 the periphery. This layer, for convenience's 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 separated 

 by fluid. The flat ectoderm cells next become long slen- 

 der 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 stead- 

 ily denser, until this region of the embryo is occupied by 

 a mass of irregularly polygonal 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 

 various shapes. The unciliated pole is made the more 

 conspicuous because of the pigment (orange) which covers 

 the rest of the embryo. In this condition, the embryo 

 breaks through its follicular wall, and passes out of the 

 parent through one of the oscula. 



The first step in the metamorphosis of the larva con- 

 sists in the flattening of its ectoderm. The flattening 

 begins before the sponge attaches itself, and gradually 

 travels from the non-ciliated or spiciilar pole backwards. 

 While there is still a considerable remnant of the colum- 

 nar ectoderm, the larva attaches itself by the spicular 

 pole, but obliquely, so that it lies somewhat on its side. 

 The rest of the ectoderm then becomes flattened, and 

 the lava is transformed into a thin, flat mass, circular in 



