MORPHOLOGY 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 



Fio 16. Early blastula stage of 

 the sea-urchin egg. 



FIG. 17. Gastrula stage of the 

 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 

 larva at the same time assumes a 

 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 



FIG. 18. The pluteus stage of 

 the larva of S. purpuratus. 



