DEVEI.OPMENT OF SPONGES 



l6l 



All sponges produce sex cells, which seem to arise from 

 amceboid mesoglcea cells retaining an embryonic character. 

 In the case of the ovum, the amoeboid cell increases in 

 size, and passes into a resting stage ; in the case of the 

 male elements, the amoeboid cell divides into a spherical 

 cluster of numerous minute spermatozoa. The similar 

 origin of the ova and spermatozoa is of interest. Most 

 sponges are unisexual, but many 

 are hermaphrodite. In the latter 

 case, however, either the produc- 

 tion of ova or the production of 

 spermatozoa usually preponder- 

 ates, probably in dependence 

 upon nutritive conditions. 



Development. — It is not sur- 

 prising to find that there is great 

 variety of development in the 

 lowest class of Metazoa ; it seems 

 almost as if numerous experi- 

 ments had been made, none 

 attended with progressive success. 



The minute ovum, without any 

 protective membrane, usually lies 

 near one of the canals, and is 

 fertilised by a spermatozoon 

 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, 



Fig. 78. — 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 (f.c.), and 

 lower hemisphere of granular cells {gr.c). 



5. Gastrula stage settled down. Ec, Outer 



layer ; En., inner layer ; bl., closing 

 blastopore ; am.p., mooring, amoeboid 

 processes. 



II 



■gf.C. 





am.p 



