142 



blastocoel contains mucopolysaccharides secreted by the blastomeres (Monne 

 and Harde, 1951). Cilia form on the blastula before it leaves the membrane; 

 each blastomere bears a single cilium. Some time after formation of cilia, the 

 blastula is immobile; then the cilia begin to move and the blastula initially 

 starts to rotate slowly inside the membrane. The blastula is capable of rotating 

 both in a clockwise and an anticlockwise direction around the animal-vegetal 

 axis. The direction of movement changes at brief halts (Fewkes, 1893). During 

 rotation the blastula veers to the margin and begins to "rub" it. The blastula 

 hatches after the 10th division due to the action of a hatching enzymes 

 secreted by the blastomeres beginning from the 8th division. After hatching, 

 the vegetal wall slightly flattens, and the region of the animal pole thickens 

 and forms an apical organ with long immovable cilia (Selenka, 1880; Okazaki, 

 1975). The blastula swims with its animal pole forward while rotating around 

 the animal- vegetal axis (Maruyama, 1981). 



Thus, in sea urchins the embryo leaves the membrane in the blastula 

 stage to commence its fi^ee-swimming mode of life. 



Gastrula 



Within a few hours after the blastula has hatched, the cells of the mes- 

 enchyme lose their cilia and begin to migrate from the region of the vegetal 

 pole into the blastocoel; these cells later produce the spiculogenous syncy- 

 tium. This is followed by gastrulation. Its mechanism in sea urchins has been 

 described in detail by Gustafson (1963, 1964, 1975). In the course of gastru- 

 lation, the cells of the vegetal pole invaginate inside the blastocoel, to form 



Figure 98; Blastula before leaving the membrane. 

 A — Scaphechinus mirabilis; B — Strongylocentrotus nudus. 



