XVI 



ECHINODEEMATA 



485 



OPHIOTHRIX FKAGILIS 



Methods of preservation and staining which are applicable to 

 the study of Asterias are equally applicable in the case of Opliiothrix, 

 and it will therefore be unnecessary to repeat them here. One 

 interesting point, however, remains to be noticed. The normal 

 development of the eggs of Opliiothrix fragilis was only obtained 



ong 



blp 



-/7.Z/ZZC 



FIG. 373. Early stages hi the development of Opliiothrix fragilis. (Original.) 



A, early blastula, seven hours old. B, gastrula, twenty-one hours old. C, Larva, thirty hours old, 

 viewed from the ventral side, arch, wall of archenteron ; a.vac, anterior vacuolated crest ; We, blasto- 

 coele ; Up, blastopore ; p.mes, primary mesenchyme ; p.vac, posterior vacuolated crest. 



in the case where these eggs were spawned naturally ; this was 

 accomplished by placing the males and females, which are easily 

 recognizable by the colour of the gonad shining through the skin, 

 in pairs, in separate vessels of clean sea-water. The females have 

 a colour which varies from light red to dark-brownish red, the males, 

 on the contrary, are of a light yellow or buff colour. The eggs are 

 very small, not more than -1 mm. in diameter, and are rendered 

 quite opaque by the dense accumulation of red yolk grains. 



