ANATOMY OF THE PEARL OYSTER. 69 



of the viscero-pedal mass (Plate IX., fig. 1). Yet although the gonads envelop the 

 viscera of this region, they do not hide the hyssal gland, which, lying excentrically, 

 comes in contact with the hody-wall on the right ; and when the viscero-pedal mass 

 is viewed from this aspect (Plate VI., fig. 2), the hyssal gland is seen as a broad hand 

 reaching from the base of the foot backwards to the right retractor muscle. This 

 band has the appearance of dividing the right gonad into a dorsal larger part and a 

 ventral smaller (Plate VI., fig. 15), a division more apparent than real, as the two 

 parts are continuous to the left of the byssal gland. No portion of the reproductive 

 glands extends into the foot proper, or into the mantle as in the case of Mytilus. 



When mature, the male and the female gonads are practically indistinguishable 

 from one another to the naked eye. Both are usually pale creamy yellow in colour ; 

 the male, in some cases, rather paler than the female. The male, too, is rather less 

 bulky than the female ; this, however, is no guide to the sex, as the bulk will in that 

 case approximate to that of a partially developed female. 



The gonads, testes or ovaries as the case may be, consist of branched tubuli, whereon 

 cluster myriads of saccate creca, the alveoli (Plate IX., figs. 21, 22). In these arise, by 

 proliferation from the germinal epithelium of the walls, spermatozoa or ova, according 

 to the sex. The accumulated ripened products filling these alveoli and tubuli are 

 then passed on into three trunks, which converge into a single main vessel just 

 within the external genital aperture (Plate VII., fig. 11). This opens immediately 

 dorsal to the renal aperture of the same side, and, indeed, the vestibule into which 

 they both open is really a deep cleft whereof the V-shaped bottom merges 

 imperceptibly into the primary genital duct. 



The spermatozoa (Plate IX., fig. 22a) are excessively minute and of the typical form. 

 The head is comparatively large, clear, and highly refractile, ovate in outline ; while 

 the long flagelluni, proceeding from its more rounded end, is from nine to twelve times 

 the length of the head. 



The ovarian ova (Plate IX., fig. 21), measuring 16 /x by 8 /*, are more or less poly- 

 gonal in form, by reason of mutual pressure, while within the alveoli and tubuli of the 

 female gonad ; but when shed they become of a laterally compressed pyriform, or ovate, 

 shape. The former is most characteristic the narrow short stalk marking what was 

 originally the place of attachment to the germinal epithelium. When fertilization 

 takes place, the stalk functions as a micropyle. The vitelline membrane enclosing the 

 coarsely-granular vitellus is thin. The nucleus is large, oval in outline, very clear, 

 and but lightly granular, in length and in breadth exceeding the half length and the 

 half breadth of the entire ovum. A nucleolus is also present, very conspicuously 

 situated within the nucleus, close to its periphery. The more detailed characters of 

 the ovum and the fertilization and early embryology will be discussed in a future 

 Part of the Report in connection with the Life-History of the Pearl Oyster. 



