with the basal membrane of the follicle wall is 

 narrowed to an elongated stem. The nucleus 

 increases gi-eatly in bulk, and the developing egg 

 assumes a pear-shaped form. An accumulation 

 of dark granules (mitochondria) at the proximal 

 end of the cells may indicate that food for the 

 growing ovocyte is obtained through the wall of 

 the follicle. The granules are not pronounced in 

 the ovocyte of C. lirginica but are conspicuous in 

 some other bivalves, particxilarly in Sphaerium 

 (Woods, 1932). From the beginning of sexual 

 differentiation to the final maturity of an ovum, 

 the early ovocyte increases in volume more than 

 3,000 times. 



Ovogenesis in the Sydney rock oyster C. com- 

 mercialis, described by Cleland (1947), is some- 

 what different from the ovogenesis of the American 

 species. At the earliest stage before the start of 

 the growth phase an ovocyte of the rock oyster 

 is a small cell, 4 m to 5 m in diameter. Two-thirds 

 of the cell is occupied by a clump of chromosomes 

 siu-rounded by a rim of clear cytoplasm. Cleland 

 identifies this stage as a definite auxocyte, i.e., 

 an ovocyte just before entry into the growth 

 phase. At ths stage the cell has no nucleolus. 

 The definite auxocyte begins to grow and passes 

 through three stages (called by Cleland Auxocyte 

 I, II, and III) which differ in size and nuclear 

 structure. Auxocyte I has a diameter of about 

 9 M. with a relatively large germinal vesicle (6 /x) 

 and a nucleolus of about 1.7 m- The nucleus is 

 centrally placed in the homogeneous cytoplasm 

 with an excentric nucleolus which is not in con- 

 tact with the nuclear membrane. Auxocyte II 

 has diameter of 12.6 m, with the germinal vesicle 

 (nucleus) about 7 n and nucleolus 2 m to 3 m- 

 The cell is usually spherical with a centrally 

 located germinal vesicle and chromosomes spaced 

 more widely than in Auxocyte I. At this stage 

 a group of granules appears at one pole of the 

 nucleus. Auxocyte III is a spherical cell 20 ti in 

 diameter, with the germinal vesicle measuring 11 

 ^ and eccentrically located nucleolus of about 

 4.2 II in diameter. The cell is separated from the 

 wall and is free in the lumen of a follicle. Pro- 

 tein granules are abundant along the periphery 

 of the cell where they are found in a matui-e egg. 

 The matm-e ovocyte of C. commercialift has a 

 diameter of about 38 m- The nucleus is large, 

 about 21 M across; the nucleolus is 4.6 /i. The 

 chromosomes are paired and are usually placed 

 peripherally in the germinal vesicle but not in 



326 



contact with the nuclear membrane. Crossing- 

 over is frequently seen at this stage but the cliromo- 

 somes are not coiled. 



SPERMATOGENESIS 



Spermatogenesis in the oyster is known pri- 

 marily from the studies by Coe (1931) on the 

 development of the gonad of young 0. lurida. 

 Comparison with the gonads of C. virginica shows 

 that there is close agreement in the general features 

 of the process in both species. Progressive stages 

 of the formation of sperm begin with the undif- 

 ferentiated gonia which line the inner wall of the 

 gonad follicles. After a large number of descend- 

 ants have been produced the spermatogonia can 

 be distinguished from the ovogonia by their smaller 

 size and position within the follicles. Ovogonia 

 lie in a single row along the wall; the primary 

 spermatogonia of C. virginica are found either 

 singly or in groups between the ovogonia lining 

 the wall and in the lumen (fig. 295). 



In the hermaphroditic gonad of 0. lurida a 

 single primary spermatogonium divides several 

 times to form a cluster of cells which become 

 separated from the folhcle wall and occupy a 

 position toward the center of the lumen (fig. 296). 



The number of divisions of spermatogonia 

 presumably depends on the amount of nourishment 

 available to the gonad. Coe estimates that in 0. 

 lurida each primary spermatogonium divides six 

 to nine times to produce a cluster of 64 to 500 

 cells. In spite of the close contact the adjacent 

 cells of the clusters are separate but are held 

 together by a dehcate noncellular secretion. Its 



Figure 295. — Portion of bisexual gonad of young C. 

 virginica. gc— genital canal; oc— ovocytes with sper- 

 matogonia filling the lumen ; spc— spermatocytes ; spt— 

 spermatids; spz— spermatozoa. Photographically re- 

 produced from Coe, 1934, fig. 5A. Highly magnified. 



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