Figure 296. — Portion of an hermaphroditic gonad of O. 

 lurida. gc — genital canal; I — indifferent cells; oc — 

 ovocytes; spc — primary spermatocytes; spc> — secondary 

 spermatocytes; spz — spermatozoa united into a sperm 

 ball. Photographically reproduced from Coe, 1934, fig. 

 5B. Highly magnified. 



chemical nature has not yet been determined. In 

 poorly preserved preparations the clusters some- 

 times have the appearance of syncytia with nuclei 

 embedded in a common matrix. In both species 

 all spermatogonia have a conspicuous nucleolus 

 and loose chronuitin reticulum. As the divisions 

 proceed the diameter of the spermatogonia di- 

 minishes from about 6m to 3m or less at the last 

 stage leading to the formation of primary sperma- 

 tocytes. In C. virginica these cells are globular, 

 each with a large nucleus resolved into slender 

 threads (spiremes). This leptonema stage is 

 frequently observed in the developing spermary 

 but the conjugation of chromosomes (synapsis) 

 has not been described with any detail. However, 

 in reference to the spermatogenesis in 0. lurida, 

 Coe (1931) states that the leptotene stage "is 

 followed by the usual process of synapsis." 



The appearance of secondary spermatocytes is 

 similar to that of the primary. In C. virginica 

 they can be distinguished by their riwlial orienta- 

 tion in the lumen and small size. Different pliases 

 of spermatogenesis in C. virginica are shown in a 

 semidiagrammatic drawing published by Coe 

 (1932a) and reproduced in fig. 297. Meiotic 

 divisions and the transformation of spermatids 

 into mature spermatozoa have not been fully 

 described for C. virginica. In a mature spermary 

 the spermatozoa are always oriented with their 

 tails toward the center of the lumen. The plioto- 

 micrograph shown in fig. 298 shows the gradual 

 increase in the number of male sex cells from the 



wall of the follicle toward the center. Successive 

 stages of the spermatogenesis of 0. lurida drawn 

 by Coe are shown in fig. 299. 



Secondary spermatocytes of 0. lurida are held 

 together in spherical masses. Close contact by 

 the spermatids continues during their transforma- 

 tion into spermatozoa; in the sperm baU of a ma- 

 ture oyster the tails radiate from the center. Each 

 sperm ball is composed of from 200 to 2,000 

 spermatozoa originating from a single spermato- 

 gonium (Coe, 1932b). During mitotic divisions 

 the "prophase, metaphase, and telophase are aU of 

 typical appearance, with a delicate spindle of the 

 usual form" (Coe, 1931). Because of the crowded 

 condition of the metaphase and anaphase plates, 

 Coe was unable to determine the chromosome 

 number which he states "is not very large." In 

 two diagrammatic drawings of spermatocyte di\4- 

 sion Coe (1931, fig. 3, E and F) figures 10 chromo- 

 some pairs. This is the most common number of 

 chromosomes found by Cleland at the two- and 

 four-cell stage of cleavage in the fertilized egg of 

 C. commercialis (Cleland, 1947). The number of 

 chromosomes seen during the cleavage of C. 

 virginica eggs is discussed later (p. 345). 



STRUCTURE OF THE MATURE EGG 



Eggs in the mature ovary of C. virginica are 

 pear-shaped and compressed. Many of them are 

 attached to the follicle wall by long, slender 

 peduncles; others are free in the lumen ready to 

 be moved to the genital canals and discharged 

 (fig. 300). The long axis of the eggs varies from 

 55m to 75m depending on their shape; the width 

 at the broadest part measures from 35m to 55/i, 

 and the diameter of the nucleus is from 25m to 40/i. 

 The oblong shape is retained for some time after 

 the discharge of eggs into water but gradually the 

 egg becomes globular and denser. Under the 

 transmitted light of a microscope the nucleus 

 appears as a large, transparent area surrounded 

 by densely packed granules (fig. 301). In a 

 globular egg the nucleus cannot be seen unless it 

 is cleared in glycerol or other clarifying reagents 

 (fig. 302). 



Eggs of oysters li^dng under marginal conditions 

 in water of salinity less than 10 %o frequently 

 become cytolyzed upon their removal from the 

 o-^'ary; the nuclei appear larger than those of 

 normal eggs. Only 1 or 2 percent of these eggs 

 is fertilizable. The delicate primary' or "vitelline" 

 membrane smTounding the unfertilized egg is 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



327 



