but the greatest bulk of a ripe gonad is more likely 

 to occur in oysters which have only one spawning 

 period per year. 



Microns 



Figure 268. — Slightly slanted tangential section through 

 the opening and adjacent portion of the ovary of 

 C. virginica. Oviduct with two egg.s at the lower right 

 side. Kidney reservoir at upper right. Ovary follicles 

 at left. Drawn semidiagramniatically from a photo- 

 micrograph of a preparation. Kahle, hematoxyhn-eosin. 



Several factors besides geography influence 

 gonadal development. The most significant are 

 temperature, depth (Loosanoff and Engle, f942), 

 salinity, available food, and pollution of oyster 

 bottoms. Many examples of the suppression of 

 gonadal growth by adverse conditions may be 

 cited. For example, oysters living in waters highly 

 polluted by various trade wastes have, as a rule, 

 poorly developed gonads. Sometimes the devel- 

 opment of the sex gland is suppressed to such a 

 degree that only traces of follicles are found in the 

 visceral mass, and the layer of the digestive di- 

 verticula is visible from the surface. These oysters 

 have the greenish or brownish coloration typical 

 for the digestive diverticula which in the sexually 

 ripe oyster is not noticeable under a thick layer of 

 gonad. Oysters with suppressed gonad develop- 

 ment are found in the waters which receive con- 

 tinuous discharge of the pollutants from pulp and 



paper mills. Similar poor oysters are frequently 

 encountered in waters of extremely low salinity or 

 from areas where salinity increases to the highest 

 Umit of tolerance (34 to 40 7oo)- The determi- 

 nation of gonadal thickness described above lacks 

 precision because variation in the compactness of 

 the gonadal tissue cannot be measured, but the 

 method is, nevertheless, useful for the practical 

 purpose of estimating expected intensity of 

 spawning. 



DETERMINATION OF VOLUME AND 

 WEIGHT OF GONAD 



The fully developed gonad is the largest organ 

 of the oyster. The ovaries or spermaries can be 

 separated from the underlying digestive diver- 

 ticula by using small curved scissors. The excised 

 pieces are weighed, and their volume is measured 

 by displacement in a simple device made from a 

 glass cylinder of appropriate dimensions (depend- 

 ing on the size of the sample) with a drain pipe 

 at the bottom and a side glass tubing of about 5 

 mm. in diameter fused to the side at an angle of 

 about 45° to record the water level. A convenient 

 water level is selected and recorded, the tissue is 

 introduced, and the water is then drained into a 

 measming vessel to the previous level. The body 

 weights of adult New England oysters with fully 

 developed gonads varied in my observations from 

 14.2 to 23.2 g. The gonads comprised from 31.2 

 to 40.7 percent of the total body weight exclusive 

 of shell. The volume of the oysters' tissues ranged 

 from 21 to 24 ml., with the gonads accounting for 

 32.8 to 33.4 percent of the total bulk. Oysters 

 selected for these measurements were of the highest 

 commercial quality and with maximum develop- 

 ment of gonads. In poor oysters with light gon- 

 adal development the proportion of the weight of 

 the gonad to body weight is only a small fraction 

 of the figures given above. 



HISTOLOGY 



The gonads of the oyster originate from a group 

 of primordial germ cells located in the mesodermal 

 band on the ventral side of the pericardium in the 

 vicinity of the visceral ganglion (Coe, 1943a). In 

 embryos of bivalves primordial germ cells are 

 identified by their relatively large size, round 

 siiape, and clear vesicular nucleus with one or two 

 nucleoH (Okada, 1936, 1939; Woods, 1931, 1932). 

 Tlie primordium soon becomes separated into two 

 groups whicli by continuous multiplication of the 



ORGANS OF REPRODUCTION 



299 



