139 



maturation divisions. Among echinoderms, the shedding of eggs which have 

 completed meiosis is observed only in sea urchins and sea lilies. 



Egg membrane : The unfertilized egg of sea urchins is surrounded by a 

 vitelline membrane adjoining the plasma membrane and a jellylike mem- 

 brane. The jellylike membrane consists of sulfomucopolysaccharides secreted 

 by the egg in the later stages of oogenesis. It usually has the same refractive 

 index as water and cannot be seen without staining. Staining reveals the 

 micropyle in the jelly like membrane, in which polar bodies can then be 

 identified (Boveri, 1901; Lindahl, 1932; Harvey, 1956; Piatigorsky, 1975; 

 Schmekel, 1975). In the sand dollars Echinarachnius brevis (Onada, 1938), 

 E. parma, and Scaphechinus mirabilis, the jellylike membrane contains pig- 

 ment cells (Chia and Atwood, 1982), has a fine fibrillar structure, and is 

 clearly visible under a microscope. The thickness of this membrane is 20-60 

 jim. 



Fertilization : As in most animals, contact of the spermatozoon with the 

 mature egg produces an acrosomal reaction in the spermatozoon and a cor- 

 tical reaction in the egg. It is generally believed that the spermatozoon can 

 penetrate the egg at any point. At the site of sperm penetration, a fertilization 

 cone is produced, which usually disappears in a few minutes. As a result of 

 the release of the cortical granules of the egg in the space between the plasma 

 membrane and the vitelline membrane, the latter separates from the egg and 

 is transformed into the fertilization membrane. The content of cortical gran- 

 ules takes part in the formation of the fertilization membrane and the hyaline 

 membrane, which is a thin transparent fibrillar structure, a few microns thick, 

 that directly borders the plasma membrane of the zygote. 



Cleavage : A definitive typical radial cleavage is observed in sea urchins 

 (Figures 95-97). In Paracentrotus lividus (Boveri, 1901) the disposition of 

 individual blastomeres is distinctly visible in the early stages of development. 

 The first two divisions of the egg occur in a meridional direction, giving rise 

 to four equal blastomeres. The third division is equatorial. After this, in 

 Paracentrotus lividus four nonpigmented animal and four pigmented vegetal 

 blastomeres are formed. Eventually, the animal blastomeres divide 

 meridionally, giving rise to eight mesomeres, while the vegetal blastomeres 

 divide longitudinally, giving rise to four pigmented macromeres and four 

 nonpigmented vegetal micromeres. Mortensen (1938) observed that in 

 Prionocidaris baculosa the fourth division occurs in the meridional plane in 

 all the blastomeres, as a result of which two coronas of eight cells each are 

 formed. In a large majority of sea urchins, cleavage proceeds according to 

 that described by Boveri in Paracentrotus lividus and the 1 6-blastomere stage 

 leads to formation of micro-, meso-, and macromeres. The fourth division is 



