THE DEVELOPMENT OF LAMELLIBRANCHS. 317 



they form a very thick layer, only those which lie near the 

 surface have room to develop. 



The beaker should now be allowed to stand for about 

 ten minutes, and in the meantime some of the eggs may 

 be picked out with a dipping-tube for examination under 

 the microscope. In using the dipping-tube, cover the 

 ' large end with the tip of the finger, and run the small end 

 down close to the bottom of the beaker, and then take the 

 finger off the top, and as the water runs in at the bottom 

 it will carry some of the eggs with it. When the tube is 

 filled, place the finger on the top again, and draw it out of 

 the water, and, holding it perpendicularly on the centre of 

 a glass slide, and taking the finger off the top, allow a 

 good-sized drop to run out into the slide. 



If things are working properly, each egg should now 

 have a number of male cells attached by their heads to its 

 outer surface, with their tails radiating from it in all direc- 

 tions, as shown in Fig. 159, and cover- 

 ing it in such numbers that the lashinjr 



o 



of their tails causes the egg to rotate 

 and move through the water. 



Fio. 159. Egg about two minutes after fer- 

 tilization; showing the irregular outline, the 

 large genninative vesicle, and the spermatozoa, 

 attached to the surface of the egg. FIG. 159. 



As soon as all the eggs have male cells attached to them, 

 it is necessary to get rid of the superfluous male fluid, for 

 it would soon decay and pollute the water if it were allowed 

 to remain, and if it is not drawn off from the eggs while 

 they are at the bottom, it is almost impossible to remove 

 it after the embryos have begun to swim, without losing 

 them as well. 



