CHAPTER XV 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



Pafje 



Ovogenesis - --- - '^^ 



Spermatogenesis -- - --- ^^^ 



Structure of the mature egg -- -- -- 327 



Cytoplasmic inclusions 328 



Structure of the mature spermatozoon -— - - 336 



Fertilization 338 



Acrosomal reaction 341 



Fertilization of egg --- --- 342 



Aging of eggs and sperm 343 



Polarity of egg 344 



Cleavage -- - 344 



Bibliography - - 361 



As early as 6 to 10 weeks after setting, young 

 C. virginica of New England waters, then 6 to 8 

 mm. in height, develop primordial gonads of pro- 

 fusely branching tubules (Coe, 1932a). At this 

 stage the germinal epithelium is a layer of morpho- 

 logically undifferentiated cells; some of them will 

 transform into larger cells to become ovocytes, 

 i.e., the cells destined to develop into mature eggs. 

 The smaller cells of the epithelium proliferate very 

 rapidly and are recognizable as the male germ line, 

 and eventually develop into spermatozoa. For 

 several weeks the immature, or primary, gonad of 

 an oyster remains nonfunctional and bisexual 

 (ambisexual), for it contains both male and fe- 

 male germ cells which will transform into mature 

 spermatozoa or ova during the following summer. 

 In some individuals the primary bisexual gonad is 

 retained until the second year, a delay which Coe 

 (1932a, 1938) attributes to poor nutrition. 



The more rapid multiplication of male germ 

 cells suppresses the development of ovocytes and 

 results in a predominance of males among the 

 1 -year-old oysters and in the appearance of dif- 

 ferent degrees of intersexuality (predominance of the 

 cells of one sex over the other) . In the same brood 

 which contains also distinctly ambisexual oysters 

 there are, however, other young individuals in 

 which the primary gonad develops directly into 

 ovary or spermary. Local conditions on oyster 

 beds apparently influence the tempo of changes. 

 In the warmer waters at Beaufort, N.C., young 

 oysters are more apt to develop directly into fe- 

 males than in the northern cold waters of New 



England. Coe (1938) found that the proportion 

 of females to 100 males varied at the first breeding 

 season between 37.1 and 48.8 at Beaufort; 5.6 and 

 24 at Milford, Conn.; and 3.3 and 12.5 at New 

 Haven Harbor. The differences are not consistent 

 with geographical latitude since the female to male 

 ratio at West Sayville, Long Island, N.Y., was 

 31.2; at Delaware Bay 41.9; and at Apalachicola, 

 Fla., 7.1. It is obvious that these variations can- 

 not be attributed to temperatm-e alone and are 

 probably caused by a combined effect of envii'on- 

 mental conditions. 



Toward the end of the second breeding season 

 the primary gonad is transformed into a definite 

 ovary or spermary (fig. 291). The gametogenesis, 

 i.e., complete transformation of the primordial 

 germ cells into mature ova (ovogenesis) or sperma- 

 tozoa (spermatogenesis), is a very complex process. 

 The differentiation is accompanied by rapid multi- 

 plication of the new generations of cells which 



Microns 



Figure 291. — Section of a gonad of C. virginica at an early 

 stage of differentiation. End of Marcli, Woods Hole, 

 Mass. The larger, clear cells are ovogonia, the smaller 

 ones are undifferentiated cells of germinal epithelium. 

 Bouin, hematoxylin-eosin. 



324 



FISHERY bulletin: VOLXJME 64, CHAPTER XV 



