SEA-FISHERIES LABORATORY. 163 



the water in the tanks was rendered quite turbid, as if 

 milk had been poured in. A drop examined under the 

 microscope was found to be teeming with spermatozoa in 

 active movement. Two days afterwards the Roosebeck 

 mussels discharged spermatozoa. From that date on- 

 wards, although no further discharge of spermatozoa was 

 observed, the eggs were alwa3^s fertilised. With a very 

 few exceptions the eggs were discharged during the night. 

 Many of the mussels were actually observed in the act 

 of shedding their eggs. 



The embryos flow from the female in a slow distinct 

 stream. When not disturbed by currents they settle 

 down on the mud close to the parent as an obvious pink 

 mass. They remain in this position undergoing the early 

 stages of development, which last from eight to twelve 

 hours. They then rise to the surface as free swimming 

 larvae, and are dispersed by the currents. The duration 

 of the free swimming stage was not determined, but the 

 larvae remained free swimmers for at least four days. 

 The minute size of the larvae (they are only from 3^3 to 

 aio of an inch in diameter) prevents accurate observations 

 being made for one individual. They can only be kept 

 in jars where no circulation is going on, consequently the 

 surroundings soon become unfavourable to life. Attempts 

 to strain them through fine sieves, and so allow the foul 

 water to escape, were not a success, as the larvae passed 

 through our finest sieves. 



Fertilisation of the eggs has generally been thought to 

 take place in the water after they were discharged from 

 the parent. That has not been our experience at Piel. 

 After the obvious discharge of sperms all the eggs sub- 

 sequently extruded were found to be fertilised, even 

 though no further discharge of spermatozoa could be 

 detected. In several instances the eggs were isolated as 



