Microns 



60 



Figure 274. — Ovarj' of C. virginica shortly after comple- 

 tion of spawning. Unspawned ova are cytolized. 

 Note the invading phagocytes and the disorganization 

 of connective tissue. Bouin, hematoxylin-oosin. 



the young sex cells are not sufficiently differ- 

 entiated and their sexes can not be identified. 

 Ovocytes, when sufficiently developed, can be 

 distinguished from spermatocytes by their large 

 nuclei and granular cytoplasm. 



During spawning the sex cells are moved inside 

 the genital canals by ciliary motion of the epithe- 

 lium. How they are released from the follicles 

 and reach the genital canals located near the sur- 

 face of the sex gland is not knowm. Histological 

 examination discloses no contractile elements in 

 the tissues surrounding the follicles, and no con- 

 traction of the gonad or part of it could be detected 

 during spawning. Upon reaching the gonoduct 

 the released cells continue to be moved by the 

 powerful cilia of the duct and vestibule (fig. 275) . 

 Longitudinal aiiu circular muscle fibers are found 

 under the basal membrane of the vestibule and 

 appear to be better developed in the male gonad 

 than in the female. Roughley (1933) refers to 

 the presence of sphincter muscles in the gonoducts 

 of Ostrea (Crassostrea) commercialis , but no struc- 

 ture resembling a sphincter is found in C. virginica 

 (Galtsoff, 193Sa). It is conceivable that a contrac- 

 tion of circular muscle fibers of the spermiduct to 

 a certain degree controls the passage of sperm 



through the genital opening, but no such action 

 has been detected under experimental conditions. 

 The epithelium of the distal end of the gonoduct 

 and of the urinogenital vestibule contains, besides 

 the ciliated cells, a large number of mucous cells 

 (MC) not present in the lining of the canals. In 

 the ovary the opening of the oviduct into the 

 vestibule is marked by a small ridge of ciliated cells 

 (Galtsoff, 1938a, 193Sb). 



SPAWNING 



The sexual apparatus of oysters is of the simplest 

 type for it lacks the accessory sex organs which in 

 some moUusks are used for mutual excitation, 

 storage of sex cells, copulation, and secretion of 

 egg capsules. In spite of the structural simplicity 

 of the reproductive organs, the spawning of the 

 female oyster is a rather complex action which in- 

 volves coordination of the gills, nervous system, 

 mantle, and the adductor muscle, while the sexual 

 act of the male is much simpler. It is, therefore, 

 more convenient to describe separately the phe- 

 nomena involved in the spawning of the two sexes. 

 Under natm-al conditions simultaneous release of 

 sperm and eggs, essential for successful reproduc- 

 tion of the species, is attained through mutual 

 stimulation. 



SPAWNING REACTIONS OF THE FEMALE 



Spawning of the female proceeds in several 

 consecutive steps which, in the order of their 

 participation, involve the ovary, the gOls, the 

 mantle, and the adductor muscle. The behavior 

 of these organs follows a distinct pattern, one 

 action succeeding the other in a precise sequence 

 which fuially termiTiates in the dispersal of eggs 

 in the surrounding water. 



The first step is ovulation, i.e., the discharge 

 of eggs from the ovary into the epibranchial 

 chamber. The moment the eggs appear in the 

 epibranchial chamber the two edges of the mantle 

 come together and effectively seal the pallial 

 cavity and cloaca. This peculiar behavior of the 

 mantle may be observed in a spawning female 

 oyster placed close to the wall of a rectangular 

 tank. In an actively feeding animal the space 

 between the two mantle edges is wide open; the 

 pallium and the tentacles are extended outward 

 parallel to the surface of the valves and the gills, 

 exposing the side of the adductor muscle, the 

 rectum, and the inner part of the cloaca (fig. 276). 

 A different picture is seen m a spawning female. 

 A few minutes before ovulation the edges of the 



ORGANS OF REPRODUCTION 



303 



