196 



Egg membrane : Oocytes are surrounded by a thick jellylike membrane, 

 which becomes loose (or disintegrates) after the oocytes are released into the 

 water (Lonning, 1976). As early as 1916, Kirk described a larva grown from 

 an egg with a dense chitinoid capsule. Unfortunately, the species affinity of 

 this larva was not determined since he had removed the eggs from the plank- 

 ton. In 1937, Mortensen described the secondary capsule, provided with spines, 

 for three species of the genus Ophiocoma: O. echinata, O. erinaceus, and O. 

 scolopendrina; he also noticed that the capsule obstructs emergence of the 

 embryo during hatching. In structure these capsules resemble the gemmules 

 of sponges or the chorion of eggs of some chitons. 



Fertilization : The eggs in species with planktotrophic or lecithotrophic 

 larvae are shed in water through a slitlike opening in the bursa, where fer- 

 tilization occurs. One of the consequences of fertilization is the completion 

 of maturation division of the egg. Olson (1942) found two polar bodies in 

 Ophiopholis aculeata, which lie above the blastomeres after the first division. 

 Hendler (1977), investigating the development of Amphioplus abditus, de- 

 scribed the formation of polar bodies that remained quite distinct during the 

 first three-four divisions. 



During fertilization, the contents of the cortical granules of the eggs are 

 released into the space between the plasma membrane of the egg and its yolk 

 membrane, which transforms into a fertilization membrane and separates 

 from the surface of the egg. At this time, a hyaline layer appears that lines 

 the plasmatic membrane of the egg (Holland, 1979). This has been reported 

 for Ophiocoma nigra (Narasimhamurti, 1933), Gorgonocephalus caryi (Patent, 

 1970a), and Amphioplus abditus (Hendler, 1977). 



Cleavage : Completely radial cleavage is observed in brittle stars. The 

 first two divisions occur in the meridional plane and the third in the equatorial 

 (Figure 147). The blastomeres in the animal hemisphere in Ophiocoma nigra 

 are somewhat smaller than those in the vegetal hemisphere (Narasimhamurti, 

 1933). Sometimes the blastomeres do not differ in size; the next division 

 gives rise to the blastula. 



Blastula 



Early development from fertilization to the blastula takes 24 hrs in 

 Ophiothrix fragilis (McBride, 1907), 21 hrs in Ophiopholis aculeata (Olsen, 

 1942), 6 hrs in Ophimaza cacaotica (Mortensen, 1937) and Ophiura sarsi and 

 2.5 hrs in Ophiothrix oerstedi (Mladenov, 1979). Mortensen (1937) noted that 

 in Ophiocoma lineolata the embryo hatches from the envelopes at the morula 

 stage. Formation of the blastocoel begins early in brittle stars. Narasimhamurti 

 (1933) noted that indications of the blastocoel appear in Ophiocoma nigra at 

 the 16-cell stage. In Ophiura sarsi an extensive primary cavity was noticed 



