1 86 BULLETIN OF THE BUREAU OF FISHERIES. 



water settle to the bottom in a pink or reddish mass. The ova, which are clustered 

 together in the shape of rods, soon fall apart and assume the normal spherical or oval 

 form. The process sometimes continues until nearly all the genital products are re- 

 moved ; sometimes the process is incomplete and occurs periodically from two to three 

 days apart; and sometimes the animals fail to expel any of the elements. In the latter 

 case the eggs and spermatozoa degenerate and are absorbed by the tissues of the body. 

 The normal stimulus which starts up the act of spawning still remains an unsolved 

 problem. Ripe specimens were transferred from cold to warm water and vice versa, 

 from water of high density to that of low density and back again ; they were exposed to 

 the air from one to three hours and then submerged, subjected to swift currents and then 

 still water, but in no case was there positive evidence that it influenced the act of spawn- 

 ing. Rough handling, such as shaking them up and down in a bucket of water or stirring 

 them about vigorously with the hand, caused spawning to take place within a few min- 

 utes to an hour later, but this can not be called a normal stimulus. It is a common 

 belief that the presence of spermatozoa in the water stimulates the female to the act of 

 spawning, but the author could not verify this statement. Often the females spawned 

 before the males began to liberate sperm, and isolated individuals in filtered sea water 

 were observed to deposit eggs in great quantities on several different occasions. On 

 the other hand, sometimes the trough in which quantities of ripe mussels were kept 

 would be milky with sperm and not a female would show a sign of laying an egg. The 

 following day, when very few or no spermatozoa were present, a dozen or more females 

 might spawn. The nature of the spawning stimulus, therefore, remains doubtful. 



EMBRYOLOGY. 



GERM CELLS. 



The early history leading up to the formation of the germ cells could not be 

 worked out from any of the author's slides, although histological preparations were 

 made from material collected every two weeks during the year. Dividing cells were 

 rarely observed, but it was clearly evident that the sexual elements arise from the epi- 

 thelial lining of a vast number of canals that proliferate throughout the whole body. 

 The germ cells begin to form early in the winter and reach maturity by early spring 

 or late summer, according to the temperature of the water and, possibly also, the 

 amount of available food. On our Atlantic coast the spawning season begins in April 

 and continues on through the summer well into September. At the Woods Hole 

 (Mass.) Biological Station, the author has secured spawning individuals every week 

 throughout the season from June 20 to September 15. 



The number of germ cells liberated is something enormous, but that is to be 

 expected when we consider that practically the whole body functions as a genital 

 gland. In figure 147 (opp. p. 163) the genital follicles filled with their products are 

 shown to occupy almost the entire portion of the mantle, the floor of the pericardial 

 region, the wedge-shaped mesosoma, and the outer walls of the liver. A male mussel 

 discharges a stream of milt that in a few minutes' time will render milky all the water 

 in a trough 10 feet long, 1% feet wide, and 3 inches deep. Figures can not be used to 

 express the countless numbers of fertilizing elements that swarm in the water under 

 these conditions. 



