filaments by microvilli issuing from the sperm acrosome. The sperm may 

 remain so attached for several months, until fertilization (O'Foighil, 1985). In 

 Crassostrea virginica (Longwell and Stiles, 1969), Ostrea rivularis (Hamada, 

 1927), and Mytilus edulis (Longo and Anderson, 1969), fertilization takes 

 place at the stage of metaphase I division of the maturing oocyte. Contrarily, 

 in Ostrea circumpicta and Spisula solidissima, the oocytes retain the germinal 

 vesicle until the time of fertilization (Hamada, 1927; Schechter, 1941; Sachs, 

 1971). According to observations by Vasetskii (1973) and Ginzburg (1974), in 

 Crassostrea gigas, Spisula sachalinensis and Mactra chinensis, fertilization 

 may take place at any stage of the process of maturation up to metaphase II. 

 According to Hylander and Summers (1977), upon activation of the sperm 

 during fertilization, the axial rod of the acrosome surrounded by the membrane 

 of the acrosomal vesicle is transformed into an acrosomal process making 

 contact with the plasmic membrane of the microvilli of the egg. 



Cleavage : The eggs of bivalves are subject to spiral heteroquadrant cleav- 

 age. The cleavage plane of the first division divides the egg into two unequal 

 blastomeres: the smaller AB and the larger CD. After the second division, a 

 four-celled embryo is formed in which blastomere D is much larger than the 

 remaining blastomeres (Figures 1 and 2). In many species, during division by 

 cleavage, a polar lobe forms; this is a large cytoplasmic process at the vegetal 

 pole which, during division by cleavage, merges with blastomere CD and in 

 the second division, with blastomere D (Figure 3). Cleavage in bivalves loses 

 its synchrony early. Blastomere D and its daughter cells divide faster than the 

 other blastomeres of the embryo. Among the daughter cells of blastomere D 

 of the main quartet, blastomeres 2d and 4d are distinguishable, which are the 

 first and second somatoblasts. In species with a polar lobe, its material first 

 merges with blastomere ID and then with blastomeres 2d and 4d. 



Cleavage in bivalves is characterized by a strict order of sequence of 

 dexitropic and laeotropic divisions. However, in many cases (for example, in 

 Crassostrea), this order is disturbed (Fujita, 1929). 



Blastomere 2d, after four small cells have separated from it, splits uni- 

 formly into the right cell Xd and the left Xs. Blastomere 4d divides into two 

 equal cells, Md and Ms (Figure 4, A). 



As a result of cleavage, a ciliated sterro- or coeloblastula is formed; this 

 is the first larval stage of bivalve moUusks which is provided with cilia. A 

 sterroblastula is characteristic of marine bivalves and a coeloblastula of brack- 

 ish and freshwater mollusks (Malakhov and Medvedeva, 1986). The 

 sterroblastular cavity is occupied by a large cell, 2d or its daughter cells 

 (Figure 4, B). In the fresh- and brackish-water mollusk Dreissena polymorpha, 

 on the other hand, the blastocoel, which performs an osmoregulatory fianction, 

 is formed very early (Meisenheimer, 1901). The primordium of the shell gland 



