39 



there had been considerable retardation ui the rate of 

 growth, or that they had resulted from spawning early in 

 the year. The early maturity of the spermatozoa is in- 

 teresting in this connection. In some specimens taken 

 in December by Mr. Scott, ripe spermatozoa, which 

 remained alive for about 12 hours, were expressed from 

 the mantle lobes. An attempt was made to bring about 

 artificial fertihzation, but although the eggs were found to 

 be covered with motile spermatozoa, and in some cases the 

 formation of the first polar body took place, the segmen- 

 tation of the ovum was not observed. 



The assumption that some Mussels at least may spawn 

 early in the year is necessary to explain the presence of 

 larvae in January and February, if it should be found 

 impossible to account for the presence of such by con- 

 siderable variation in the early development and rate of 

 growth ; and that there is a gradual emission of spawn 

 during April, May, and June seems probable also in view 

 of the fact that free-swimming larvoe are to be found in 

 May and June. But from the anatomical standpoint, 

 such secondary spawning periods are accidental, and do 

 not effect the statement that there is a yearly cycle of 

 changes in the reproductive organs of the Mussel, which 

 begins with the termination of the act of spawning some- 

 time in the summer months, includes the gradual and 

 continuous ripening of the gonads, and ends with a com- 

 paratively rapid extrusion of the reproductive products, 

 leaving the animal in a " spent " condition, after which 

 a short period of rest occurs, and the cycle of changes is 

 repeated. The duration of the maximum spawning period 

 we have not precisely made out, and its date is probably 

 variable and dependent on changing meteorological con- 

 ditions, but that it occurs during May, June, July, and 

 August seems perfectly certain. This cycle of changes 

 will now be considered. 



