ON EGGS OF MARINE ANIMALS 29 



One final word which concerns all these species of starfish: 

 the optimum moment for fertilization comes during the stage of 

 the eggs' first maturation. Eggs inseminated in the stage of the 

 intact germinal vesicle never leave this stage. After separation 

 of the first polar body, fertilization capacity begins to fall off; 

 although fertilization is possible even after complete maturation, 

 neither it nor later development is normal. 



Thy one 



As far as I know, Pearse was the first to observe the shedding 

 of eggs by Thyone in the Marine Biological Laboratory, Woods 

 Hole. I have during several seasons obtained eggs in optimum 

 condition for fertilization by allowing the animals to shed. For 

 many workers this may not be a good egg because of its opacity. 

 It has nevertheless some interesting points. Thyone is extremely 

 abundant at Woods Hole. 



Synapta 



At Roscoff I have studied eggs of Synapta shed in the 

 laboratory. 



Nereis limbata 



Unlike the forms that I have so far considered in this section, 

 Nereis limbata is sexually dimorphic. The males are bright red 

 anteriorly with white posterior segments, the females pale yellow 

 or light green. 



The animals are caught after sunset on certain nights, with 

 a few exceptions, during the "dark of the moon" in the months 

 of June, July, August and September. (Lillie and Just, 1913.) 

 In Woods Hole, the most favorable locality for collection is the 

 float stage in the Eel Pond back of the Supply Building. The 

 worms appear swimming near the surface of the water about an 

 hour after sunset. Attracted by the light of a lantern or an 

 electric light (in Woods Hole, the float stage has been wired and 

 two electric plugs are to be found in a box attached to the boat- 

 shed) the worms are readily caught with a hand net. In general 

 the swarming begins with the appearance of a few males swim- 

 ming rapidly in curved paths in and out of the circle of light cast 

 by the lamp. The much larger females then begin to appear. 



