106 



surface of the egg in a few minutes and enlargement of the perivitelline space 

 continues up to commencement of cleavage (Kume and Dan, 1968). 



Cleavage : About 1 - 2 hr after fertilization, cleavage begins, which is 

 usually radial in echinoderms. The first two cleavage ftirrows are meridional 

 and the third equatorial (Figure 65). The somewhat unequal size of the first 

 blastomeres in some species of sea stars, indicates weakly expressed bilateral 

 cleavage (Field, 1892; Gemmill, 1914; Kasyanov, 1977). Although the first 

 cleavages in general are synchronous, strict synchronization of cleavage is not 

 observed. In sea stars the blastomeres are usually very loosely bound to each 

 other right up to the formation of the coeloblastula. The characteristic divi- 

 sion into macro-, meso-, and micromeres of the sea urchin embryo is absent 

 in sea stars. 



As a result of cleavage, a morula or blastula with rather loosely bound 

 blastomeres forms (Figure 66) and then a coeloblastula with an extensive 

 blastocoel and firmly adhering blastomeres (Dan-Sokhawa, 1977). The ab- 

 sence of completely radial division is a distinguishing feature of Fromia 



Figure 65: Cleavage of Patina pectinifera into four blastomeres. 



^ ^.^ .^ ' B 



Figure 66: Blastula of Patiria pectinifera. A — blastomeric blastula; B — coeloblastula. 



