Microns 



20 



Figure 328. — Early stage of gastrulation in the egg of 

 C. virginica. Seen at a section of an egg cut along its 

 axis. Heidenhain, iron-hematoxvlin. 



found that equal cleavage can be induced arti- 

 ficially in eggs of the Japanese oyster by cen- 

 trifuging for 2 minutes at 1,500 r.p.m. and at the 

 centrifuge radius of 14 cm. 



Fujita (1929) gives a brief account of the cell 

 lineage of the eggs of C. gigas and states that the 

 mode of cleavage of this species is identical to 

 that of C. mrgmica. The main features described 

 by him are as follows. The first polar body in 

 the fertilized egg of C. gigas appears 15 minutes 

 after insemination. At the two-cell stage (fig. 

 329) the two blastomeres of unequal size, AB and 

 CD, are separated along the meridional plane. 

 Their position corresponds to the anterior (Ant.) 

 and posterior (Pst.) ends of the embryo. The 

 second division, also meridional, separates the 

 four blastomeres A, B, C, and D (fig. 329b). 



A and B represent the anterior, and C and D 

 the posterior hahes of the embryo, while B and 

 C form its left and A and D its right halves 

 (fig. 329b) . The ensuing cleavage starts with the 

 blastomere B and proceeds in laeotropic order to 

 C, D, and A; tlie resulting daughter cells, the 

 micromeres a,, b,, c,, and di, retain the shape of 

 the mother cells but are smaller. The macromere 

 D and micromere d, are respectively the largest. 

 The four daughter cells aj through d, form the 



first quartet of micromeres located between the 

 macromeres on the dorsal side of the embryo. 



The 12-cell stage is initiated by the division of 

 the macromere D; the ensuing larger cell da 

 (fig. 329c) is generally known as the first somato- 

 blast X. (In the system of nomenclature used 

 by American and European embryologists (see 

 p. 346) the ID cell gives rise to 2d and 2D and the 

 2d is the X cell.). 



The cleavage is continued laeotropically, and 

 the second generations of micromeres as, bj, 

 and C2 are smaller than the first macromeres. 

 They lie on the outside of the macromeres. The 

 third cleavage of macromeres A, B, and C con- 

 tinues in laeotropic order and results in the 

 micromeres aj, bj, and Cj; they are larger than 

 other micromeres. After the third cleavage the 

 macromeres make no further contribution to the 

 formation of micromeres and in the course of 

 development become the entoderm. The first 

 somatoblast (X cell) gives rise to many organs of 

 ectodermal origin. At the 18-cell stage of the 

 embryo the position of cell X and its first divisions 

 mark the beginning of the transition from spiral 

 to bilateral symmetry (fig. 329d). 



The mesoderm begins to form at about the 

 32-cell stage with the appearance of cell 4d, 

 the second somatoblast, also designated as cell 

 M. In bivalves the cell M remains at the surface 

 for a long time, then divides into the two meso- 

 dermal teloblasts which sink into blastocoel 

 (Raven, 1958, p. 117). The formation of meso- 

 derm in C. gigas has not been followed in detail, 

 but as a rule the mesoderm bands in bivalves 

 remain rather rudimentary (Raven, 1958). Fujita 

 states that the establishment of the three germinal 

 layers in C. gigas is completed at the 30-cell 

 stage (fig. 329 e and f). 



The gastrula stage is reached in 4 to 6 hours. 

 The cell lineage of C. gigas is generally comparable 

 to that described for other bivalves (see: Raven, 

 195S, p. 70), but for details the reader should 

 consult Fujita's (1929) original text and his 

 drawings. 



In about 4 to 6 lioui-s after fertilization, an egg 

 of C. virginica reaches the stage (fig. 330) when 

 a few large cilia become nsible at the vegetal 

 pole, the oval-shaped body is covered with very 



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