226 



(Chia and Buchanan, 1969). The presence of a micropyle process has been 

 reported at the animal pole of the oocyte, which is connected with the fol- 

 licular cells. 



Fertilization : In holothurians extrusion of gametes in water as well as 

 their release in the coelomic sac has been observed, for example in Chiridota 

 rotifera (Clark, 1910). The spermatozoa penetrate not the mature egg but the 

 oocyte. When a micropyle is present, the sperm reaches the oocyte through 

 it. Ohshima (1925) observed in Thyone briareus sperm penetration at the 

 equator from the side of the vegetal hemisphere. During fertilization an 

 acrosomal reaction of the sperm occurs and a distinct fertilization cone is 

 formed (the primary eminence) in the eggs of Holothuria atra and Thyone 

 briareus (Colwin and Colwin, 1957). 



After sperm penetration, meiotic division of the oocyte is completed with 

 the formation of polar bodies. Inaba (1930) observed in Caudina chilensis, in 

 the second meiotic division, the formation of three polar bodies which were 

 retained at the animal pole at the beginning of cleavage. 



Cleavage : In holothurians, except for Cucumaria glacialis, independent 

 of the quantity of yolk in the egg, a complete uniform cleavage of the radial 

 type is observed. The first two divisions are meridional and the third equa- 

 torial. Thereafter, meridional and equatorial divisions alternate (Figure 173). 

 As the number of blastomeres increases, they shift to the animal and vegetal 

 poles, but the blastocoel remains open for some time in the polar region (Chia 

 and Buchnan, 1969). After formation of the 32-cell stage, the time of division 

 for different blastomeres differs and their radial disposition is disturbed (Kume 

 and Dan, 1968). In Thyone briareus (Ohshima, 1925), Cucumaria echinata 

 (Ohshima, 1921), and Caudina chilensis (Inaba, 1930), features appear during 

 division which are characteristic of spiral division. A unique division is 

 observed in Cucumaria glacialis in which the eggs are large and rich in yolk. 

 As reported by Ohshima, (1931), citing Mortensen, in the holothurian initially 

 only the nucleus divides and the newly formed nuclei are situated along the 

 periphery of the egg; thereafter division of the cytoplasm occurs with the 

 formation of blastomeres. 



Blastula 



According to Mortensen (1938), in Ophiodesoma grisea the blastula is 

 formed 4 hrs after fertilization. It takes 12-14 hrs before the blastula is fiilly 

 formed. In most species the blastula is spherical and the large blastocoel has 

 a cover of cilia (Figure 174). For several species (Synaptula vittata, 

 Ophiodesoma grisea, Holothuria spinifera, and H. papillifera) Mortensen 

 (1937,1938) reported that despite the presence of cilia, the blastula remains 

 inside the envelope. Usually, hatching takes place at this stage. 



