No. 19.] 



ECHINODERMS OF CONNECTICUT. 



101 



to 



FIG. 15. Cleavage of egg of Sea-urchin, i, egg from ovary; 2, 

 mature egg with the two minute polar bodies; 3, fertilization; 4 to 6, 

 first cleavage, forming two blastomieres ; 7, 8, second cleavage, with 4 

 blastomeres; 9, 10, third cleavage, with 8 blastomeres; n, fourth cleavage, 

 with 16 blastomeres; 12, section of blastula. The blastula soon becomes 

 covered with cilia and swims actively about in the water. (After Wilson.) 



