DEVELOPMENT. 



the latter case, however, either the production of ova or the 

 production of spermatozoa usually preponderates, probably 

 in dependence upon nutritive conditions. 



Development. -It is not surprising to find that there is 

 great variety of development in the lowest class of Metazoa ; 

 it seems almost as if numerous experiments had been made, 

 none attended with progressive success. 



The minute ovum, without 

 any protective membrane, usu- 

 ally lies near one of the canals, 

 and is fertilised by a spermato- 

 zoon borne to it by the water. 

 It exhibits a certain power of 

 migration, as in some Hydroids. 

 Previous to 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. 



FIG. 61. Development of Sycandra 

 raphanus. After F. E. Schulze. 



1. Ovum. 



2. Section of i6-cell stage. 



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



lower pole. 



4. Free-swimming amphiblastula, with 



upper hemisphere of flagellate cells 

 C/.c.), and lower hemisphere of granu- 

 lar cells (gr.c.). 



5. Gastrula stage settled down. c., outer 



layer; En,, inner layer; /., closing 

 blastopore ; am.p., mooring, amoeboid 

 processes. 



