MorpHoutoGy OF DEVELOPMENT 23 
by means of which the blastula swims about. At this period, 
the membrane is burst by some unknown influence, and the 
blastula, which at first swam around within the egg sheath 
Fic 16.—Early blastula stage of Fie. 17.—Gastrula stage of the 
the sea-urchin egg. sea-urchin egg. 
(fertilization membrane), now moves about freely in the water. 
The larva very soon rises to the surface. The next step in the 
development is the gastrula stage 
(Fig. 17). On one side the cells 
grow into the blastocele and this sac 
or tube growing into the cavity is 
the rudiment of the gut. On each 
side of the gut can be seen indicated 
the rudiment of the skeleton in the 
form of two crystals or triasters. 
The further developmental stages 
consist in the organization of the gut 
into further divisions, and the out- 
growth of the triasters into a larger 
skeleton. The originally spherical 4,4 13 The pluteus stage of 
larva at the same time assumes a ‘2? !8rv2 Of S. Ppurpuratus. 
pyramidal form. It is called a pluteus (Fig. 18). 
If the larvae are not fed, they live some 14 to 18 days at a 
temperature of about 15°. The further rearing is very tedious 
