Figure 1: Cleavage in Dreissena polymorpha Pail, (from Meisenheimer, 1901). 

 A — stage with two blastomeres; B — with four blastomeres; pb — polar body. 



is formed as a result of multiplications oiXd and Xs cells (Figure 5). In marine 

 bivalves the primordium of the shell gland is invaginated in the embryo from 

 the very beginning. In freshwater forms it initially lies in the blastula wall and 

 then invaginates in the embryo (Lillie, 1895; Meisenheimer, 1901). 



Cells Md and Ms each produce one entodermal cell (enteroblast) and then 

 transform into mesodermal teloblasts from which the transitory mesodermal 

 band originates later. Some part of the mesenchyme probably originates from 

 the ectodermal cells (Meisenheimer, 1901). A notable contribution to the study 

 of the early development of bivalves is the paper by Gustafson and Reid 

 (1986). They have described the development of a primitive protobranch bi- 

 valve, Solemya reidi. Cleavage of the large (270 jim in diameter) eggs of this 

 species is to date the only example, possibly, for bivalve moUusks of an initial 

 homoquadratic spiral cleavage with equal-sized blastomeres. 



Incomplete division has not been observed in bivalves. 



Gastrulation : Gastrulation occurs through the invagination of the ento- 

 derm near the vegetal pole of the embryo. After eversion of the shell gland, 

 the blastopore shifts far forward on the ventral side of the embryo. 



Trochophore 



In marine bivalves already in the blastula (sterroblastula) stage, one or 

 several circlets of cilia appear and at the animal pole — cilia of the aboral 

 organ. Thus, in marine bivalves the blastula stage is a transition to free living. 

 Free living is also possible for stages with an everted rudimentary shell gland 

 and a well-formed blastopore. After eversion of the rudimentary shell gland, 

 a trochophorelike larva forms (Figure 6). In the fresh- and brackish-water 



