Figure 2S4. — Portion of bisexual gonad in young male 

 C. virginica. gc — genital canal; oc — ovocytes; spt — 

 spermatid; spc — primary spermatocyte; spz — sperma- 

 tozoa. Photographically reproduced from Coe, 1934, 

 fig. 5. 



(fig. 284). At this stage the C. virginica gonad 

 resembles that of 0. lurida at the completion of 

 the male phase and transition to female (fig. 285). 

 In C. virginica the spermatogonia proliferate more 

 rapidly than do the ovogonia and soon the young 

 gonad attains a predominantly male appearance. 

 Variation in the rhythm of gonad development 

 in the oysters from different localities and even 

 among those occupying the same bed residts in 

 different "categories" or "phases" of maleness 

 or femaleness. 



Development of the primary bisexual gonad in 

 young C. virginica in New England waters is 

 checked by the approach of winter when the 

 growth of ovocytes is inhibited while the number 

 of spermatogenic cells increases. A small number 

 of spermatids may be formed early in November 

 when the oysters are about 4 months old. The 

 spermary of these secondary males contain scat- 

 tered ovocytes, many of which degenerate, but 

 some of them continue to develop into ova capable 

 of fertilization. Even at the stage designated by 

 Coe (1934) as "true male" the sperms^ry at sexual 

 matm'ity still retains a small number of ovocytes 

 on the walls of the follicles. 



At the close of the first breeding season many 

 undifferentiated cells remain in the gonad to form 

 the germinal cells of the following year. 



Transformation of a bisexual gonad of C. vir- 

 ginica into an ovary begins before the formation 

 of spermatozoa. At this stage the spermatogen- 

 esis is inhibited by the growth of ovocytes and the 

 female phase is attained in a certain percentage of 

 young oysters. The protandry, i.e., the develop- 



ment of maleness before the female phase, is well 

 pronounced in C. virginica. 



At their first breeding season yoimg oysters form 

 several sex classes: immature individuals in which 

 the sex cells have not differentiated; males; her- 

 maphrodites in which functional spermatozoa and 

 ova are foimd in the gonad; and females. Her- 

 maphroditic oysters are capable of self-fertilization 

 and produce apparently normal larvae. The rela- 

 tive abundance of different sex phases of young 

 oysters varies greatly, as can be seen from table 34, 



Table 34. — Frequency of different sex phases among C. 



virginica at first breeding season 



[According to Coe, 1934] 



Figure 285. — Transition from male to female phase in 

 0. lurida. Lower left — genital canal filled with sperm 

 balls ready to be discharged; spermatogonia on the sur- 

 face of large ovocytes. Right — advanced male phase in 

 an older oyster; ovarian follicle is packed with cells in 

 the later stages of spermatogenesis. Upper left — female 

 phase preceding ovulation; many spermatogonia in the 

 lumen of the follicle. Redrawn from Coo, 19.32c. 



ORGANS OF REPRODUCTION 



315 



