43 



and a corresponding increase in mass of the parenchy- 

 matous tissue lying between the tubules. Many of the 

 branches of the tubular gonad disappear entirely, but 

 many others, and particularly the ciliated, non-glandular 

 portions, persist in situ. At the end of this period, when 

 the animal again enters on Stage I., the parts of the body 

 formerly occupied by the swollen genital tubules are now 

 the seat either of a massive syncytial tissue, or of a delicate 

 reticulum, the difference depending probably on the con- 

 dition of nutrition obtaining on the bed from which the 

 specimen was taken. 



We may now consider the characters presented by the 

 animal in the various stages referred to more particularly. 



Stage I. (PI. II., figs. 3, 4, 6). — Externally there may 

 be nothing to indicate that the specimen under examina- 

 tion has spawned, although in a section, or in a cleared 

 preparation, the difference between this stage and the 

 preceding one is striking. The sex of the animal is 

 determinable only with some difficulty, by the presence of 

 stray masses of ova or spermatozoa, which have failed to be 

 extruded. Where the germinal epithelium can be recog- 

 nized, it is not very different in male and female. The 

 area in a section of the mantle now occupied by the 

 tubules of the gonads is so greatly reduced that, on a 

 superficial examination, it might be thought that the 

 latter had completely disappeared. But more careful 

 scrutiny reveals the presence of many tubules in a col- 

 lapsed condition (PI. II., figs. 3, 4, tub. ov.), the walls 

 pressed against each other by the pressure of the reticular 

 tissue. It is, however, only the larger tubules which so 

 persist ; the finer branches have been absorbed or broken 

 down in some way. 



The space occupied in the ripe Mussel by the gonads is 

 now filled up by a large celled parenchymatous tissue 



