141 



Figure 97: Stage of 32 blastomeres: view from the animal pole. 



A — Scaphechinus mirabilis; B — Strongylocentrotus intermedius Optical section. 



Legend same as in Figure 96. 



In subsequent divisions the differences in size of blastomeres gradually dis- 

 appear (Selenka, 1880; Fewkes, 1893). In sea urchins with direct develop- 

 ment, micromeres are not formed during division and the fourth division is 

 uniform (Raff, 1986). 



Blastula 



In many species of sea urchins the blastula is formed on the first day of 

 development. According to Onoda (1938), blastula of Astriclypeus manni 

 hatches 4.5 hrs after fertilization. According to our data, in Scaphechinus 

 mirabilis the blastula also leaves the membrane after 4.5 hrs. In urchins of 

 the genus Strongylocentrotus, the blastula appears after 10 hrs in S. nudus 

 and S. intermedius (Buznikov and Podmarev, 1975) and after 13 hrs in S. 

 pulcherrimus (Onoda, 1936). The slowest blastula development occurs in 

 Scaphechinus griseus (16 hrs) and Echinocardium cordatum (28 hrs), though 

 for the latter species McBride (1918) has given a duration of 10 hrs. 



Possibly, this is because he investigated the development of E. cordatum 

 on the coast of Scotland, where the species spawns at much lower tempera- 

 tures than in the Sea of Japan, where we conducted our observations. On the 

 average, the development of sea urchins from fertilization to the blastula 

 stage takes 5-8 hrs (Onoda, 1931, 1938; Aiyar, 1936). The blastula is 

 spherical and possesses an extensive blastocoel (Figure 98). In most species, 

 in the early blastula stage the contact between the blastomeres is relatively 

 weak. Only in the apicolateral areas they are joined by desmosomes. Later, 

 the blastomere contact strengthens and the wall of the blastula consists of 

 densely arranged cells. The inner surface of these cells, facing the blastocoel, 

 is lined with a thin 0.1 fi,m layer of protein and polysaccharide material, 

 which had the function of the basal membrane (Okazaki, 1975). The 



