° Millimeters *-*•' 



Figure 275. — Section through the lowest part of spcrmiduct and the adjacent portion of the vestibule. Both organs are 

 grossly distended by the discharged sperm. Tissue of C. virginica was preserved during the act of spawning. G — 

 approximate position of the junction of spermiduct and the vestibule; CE — ciliated epithelium; CT — connective 

 tissue; I\I — longitudinal muscle fibers; MC — mucous cell; SB — opening of the vestibule into the epibranchial chamber; 

 U — lower part of the urinogenital vestibule. Bouin, hematoxylin-eosin. From GaltsofF, 1932. 



Eggs trapped inside the epibranchial chamber 

 liave to pass througli the water tubes in order to 

 accuimilate in the space between the gUls and the 

 mantle since tliere is no other way by which they 

 can reach this area (Galtsoff, 1938a). This 

 conclusion was confirmed by microscopic exami- 

 nation of a section of the gills of a female preserved 

 during tlie act of spawning (fig. 279). Any other 

 minute particles suspended in water pass through 

 tlie ostia and water tubes into tlie exhalant cham- 

 bers and are swept by the outgoing current. The 

 eggs released tlu-ough the genital pore, however, 

 take the opposite course when they enter the 

 water tubes of the gills. 



While the eggs pass through the gills the ostia 

 are wide open and the ciliary currents along the 

 filaments are neither inhibited nor reversed. The 

 eggs, therefore, flow against the current produced 



mantle display unusual activity; they come 

 together and temporarily close the access to the 

 gills; then for a few seconds they open again. 

 Both the rate and range of shell movements at 

 this time gradually increase. Finally, the en- 

 trance to the gill cavity closes completely except 

 for one small opening or "window" as shown in 

 fig. 277. Soon a white cloud of unfertilized eggs 

 appears at the window, the adductor muscle 

 contracts sharply, and the eggs are discharged and 

 dispersed several inches away from the oyster 

 (fig. 278). The opening between the edges of the 

 mantles may be formed at any place along their 

 periphery but once formed its position remains 

 unchanged throughout the duration of spawning. 

 Spawning may last from a few minutes to nearlj' 

 1 hoirr, depending on the amount of mature eggs 

 in the ovary. 



304 



FISH AND WILDLIFE SERVICE 



