128 PHYLUM PORIFERA SPONGES. 



fertilisation, the usual extrusion of polar bodies has been 

 observed in a few cases, and is doubtless general. Seg- 

 mentation is total and usually equal, and results in a 

 spherical or oval embryo more or less flagellate. This 

 leaves the parent sponge, swims about for a time, then 

 settles down, and undergoes a larval metamorphosis often 

 difficult to understand. It is peculiarly difficult to bring 



the history of the germinal layers 

 in sponges into line with that 

 in other Metazoa. 



(a] In the small calcareous sponge 

 Sycandra mphamis (Fig. 58), as 

 described by F. E. Schulze, the seg- 

 mentation results in a hollow ball of 

 cells the bJastnla. A few cells at 

 the lower pole remain large, and are 

 filled with nutritive granules ; the 

 other cells divide rapidly and become 

 small, clear, columnar, and flagellate. 

 The large granular cells become tem- 

 porarily invaginated, forming what is 

 called a "pseudp -gastrula." This 

 leaves the parent and the granular 

 cells right themselves, forming the 

 posterior hemisphere of the embryo, 

 now called an amphi - blastnla. It 

 swims for a time actively, but the 

 flagellate cells of the anterior hemi- 

 sphere are invaginated into or over- 

 grown by the large granular cells, 

 and thus what is generally called the 

 gastrula stage results. This soon 

 settles down, on rock or seaweed, 

 with the blastopore or gastrula mouth 



FIG. 58. Development of Sycandra 

 raphanns. After F. E. Schulze. 



1. Ovum. 



2. Section of 16 cell stage. 



3. Blastula with 8 granular cells (gr.c.) 



at lower pole. 



4. Free-swimming amphiblastula, with 



upper hemisphere of flagellate cells 

 (f.c.), and lower hemisphere of gran- 

 ular cells. 



5. Gastrula stage settled down. EC. , 



outer layer; En., inner layer; bl. , 

 closing blastopore ; am. p., mooring, 

 amoeboid processes. 



