DEVELOPMENT OF SPONGES. 123 



history of the germinal layers in Sponges into line with that 

 in other Metazoa. 



(a) In the small calcareous sponge Sycandra raphanus, as described by 

 F. E. Schulze, the segmentation results in a hollow ball of cells the 

 blastula. 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 



temporarily invaginated, form- 

 ing what is called a "pseudo- 



^0j^^ gastrula" This leaves the 



; fE; parent and the granular cells 



$ right themselves, forming the 



posterior hemisphere of the 



^^^^^ embryo, now called an amphi- 



blastula. It swims for a time 



xf^y^x actively, but the flagellate cells 



of the anterior hemisphere are 

 invaginated into or overgrown 

 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 downwards, and 

 is moored by amoeboid pro- 

 cesses from the granular cells, 

 which likewise obliterate the 

 blastopore. The granular cells 

 lose their granules, for the larva 

 is not yet feeding ; the now in- 

 ternal flagella disappear in the 

 absence of the stimulating water ; 

 a mesoglcea with spicules begins 

 to be formed between the inner 



FIG. 34. Development of 

 Sycandra raphanus. (After F. 

 E. SCHULZE.) 



1. Ovum. 



2. Section of 16 cell stage. 



3. Blastula with 8 granular cells 



(^r.c.) at lower pole. 



4. Free swimming amphiblastula, 

 En with upper hemisphere of 



flagellate cells (f.c.\ and 

 lower hemisphere of granu- 

 ' ar cells. 



5. Gastrula stage settled down. 

 ^^ EC. , outer layer (ectoderm ? ) ; 



En. , inner layer(endoderm ? ) ; 

 bl., closing blastopore; am.p., 

 mooring amoeboid processes. 



