110 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



As indicated above, the cycle of changes in the gonads 

 is a yearly one, or approximately a yearly one, variations 

 due to the nature of the season doubtless taking place. 

 This cycle of changes may be represented by four stages. 

 In giving the dates, we refer more particularly to what was 

 found during the year 1898. 



Stage I. — The end of July and the beginning of August. 

 The Mussel is " spent," that is, the reproductive products 

 have been entirely extruded, and in the mantle lobes and 

 visceral mass the genital tubules are, to a large extent, 

 collapsed or degenerated. The state of the animal in 

 respect of its reproductive function is one of rest. 



Stage II. — September. The gonads have begun to 

 invade the mantle lobes and other parts of the body. 

 Proliferation of ova and spermatozoa from the germinal 

 epithelium is in progress, and goes on slowly and continu- 

 ously from now till early in the following year, when it 

 becomes very active, and the mass of the gonads becomes 

 much greater. 



Stage III. — April. The mantle lobes have attained 

 their maximum of thickness, and are completely filled up 

 by the gonads. The tubules composing the latter have 

 increased greatly in sectional area, and are completely 

 filled up by ova or spermatozoa in female and male respec- 

 tively. Proliferation from the germinal epithelium is not 

 now so active as in the earlier part of the year, and 

 maturation of the genital products probably goes on. 

 Where the ova or spermatazoa are in contact with the 

 ciliated portions of the tubules, they are probably being 

 swept away and removed out of the body. 



Stage IV. — July and early August. Spawning in the 

 sense of a complete removal of the genital products is 

 now in progress. There is a rapid decrease in the mass 

 of the gonads, both in visceral mass and mantle lobes, 



