49 



reduction of the other tissues in the mantle, was obtained 

 in the specimens taken during April. Some taken later 

 were, indeed, less advanced, but generally this seems to be 

 the time at which proliferation from the walls of the 

 tubules mainly ceases. In July, Mussels were taken in 

 which spawning was in progress. In these the females 

 had almost completely spawned, and in the males the 

 gonads had undergone a great reduction in mass. A few 

 stray eggs lay scattered over the section situated in tubules 

 of greatly reduced diameter, one egg being present, as a 

 rule, in the cross section of the tubule, and generally 

 nearly filling it. Many tubules were completely empty. 

 The germinal epithelium persisted in some of these tubules 

 as one or more layers of small, rounded cells, but more 

 often its structure was difficult to make out. 



The reticulum so apparent in Stage I. is already formed, 

 but in structure is more spongy than in the stage following 

 the complete extrusion of the ova, and the spaces between 

 the bars are about equal to those occupied by the tissue 

 itself. These interspaces have, in many cases, a cir- 

 cular outline, and probably represent the former situation 

 of ovarian tubules. The general histological characters of 

 the tissue are similar to those described in Stage I., except 

 that the vacuolated appearance is not so apparent. 



The most striking changes have occurred in the male. 

 In some of the specimens taken at this time extrusion of 

 the spermatozoa has been almost completely effected, but 

 in others the animal had been taken in the act (PI. 11. , 

 fig. 5). Here the sperm tubules have been greatly con- 

 tracted in cross area, and the mass of spermatozoa (PI. II., 

 fig. 5) within the tubule has become correspondingly 

 denser. The reticulum has increased enormously, and as 

 it seems difticult to derive this tissue from the exceedingly 

 fine fibrous network present in the last stage, it is possible 



