DEVELOPMENT. 131 
the latter case, however, either the production of ova or the 
production of spermatozoa usually preponderates, probably 
in dependence upon nutritive conditions. 
Development.—lIt 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. 
Fic. 61.—Development of Sycandra 
raphanus.—After F. E. Schulze. 
x. Ovum. 
2. Section of 16-cell stage. 
3. Blastula with 8 granular cells (gxc.) at 
lower pole. 
4. Free-swimming amphiblastula, with 
-upper hemisphere’ of flagellate cells 
(fc.), and lower hemisphere of granu- 
lar cells (gv.c.). 
5. Gastrula stage settled down. c., outer 
layer; Zw, inner layer; 42., closing 
blastopore ; a@7.Z., mooring, amceboid 
processes. , 
