THE GENERATIVE ORGANS OF THE OYSTER. 277 



2. Tlie genital gland is not a compact organ situated in a definite 

 position. It is spread out superficially so as to cover nearly the 

 wliole of tlie trunk properly so called. At a short distance from the 

 surface of the body, separated from the integument by a thin sheet 

 of connective tissue, the genital gland spreads out as a system of 

 canals which unite secondarily with one another, and their internal 

 walls give rise to follicles placed vertically to the surface of the 

 body and buried in the connective tissue. 



3. The generative products are developed from the walls of the 

 follicles. Spermatozoa and ova are developed alongside of one 

 another in the same follicle. The two generative products are 

 developed in all probability from the epithelial cells, which must be 

 considered as the derivatives of the epithelial cells lining the walls 

 of the genital ducts. 



4. It is probable that a single epithelial cell is metamorphosed 

 into a single ovum, whilst only a portion of a single epithelial cell 

 becomes the mother-cell of spermatozoa. All the spermatozoa derived 

 from a single mother-cell are united into a bunch of characteristic 

 shape. 



5. The ducts of the genital gland communicate directly or indi- 

 rectly on the two sides of the body with a principal efferent duct, 

 which opens to the exterior at the anterior end of an open groove, 

 running along the muscle of the valves at a short distance from the 

 great posterior branchial nerves. 



B. Physiological. 



1. An oyster with spat in its branchige is the mother of that spat. 



2. At the moment of extrusion the ova are not only fertilized, but 

 have already passed through the earlier stages of segmentation. 



3. The spermatozoa necessary to fertilization do not come from 

 the mother oyster itself. 



4. The water which passes over the oysters brings the spermatozoa 

 which have been emitted by other oysters. Some of this enters the 

 mantle cavity, penetrates to the genital aperture, traverses it, and 

 spreads not only into the principal duct of the genital aperture, but 

 also into its larger branches. 



5. The oysters of the East Schelde may have spat in their gills 

 when two years old. Ordinarily oysters in spat are older. Oysters 

 of four or five years produce the most spat. 



6. Similarly two-year oysters may produce spermatozoa, but the 

 larger part is derived from older oysters. Neither earlier researches 

 nor my own have established with certainty that year-old oysters from 

 the East Schelde emit spermatozoa. 



