260 JOHN W. SCOTT. 



time in my experiments it was thought probable that ciliary 

 action produces a similar effect in separating eggs and corpuscles 

 during oviposition. This will be discussed later. 



3. While the egg increases in density up to the time of ovi- 

 position it decreases slightly after fertilization, the decrease 

 continuing at least as far as the trochophore stage. 



4. The comparative lightness of the sperm is undoubtedly 

 useful as an adaptation for scattering them at the time of fertili- 

 zation. Their density is just sufficient to tend to keep them near 

 where the eggs are to be found, but not enough to prevent a 

 certain amount of locomotion. 



DIRECT OBSERVATIONS. 



During the past summer I was able to make a series of direct 

 observations upon the separating process. In these observations 

 I saw the ripe eggs pass over the fimbriated membrane, along 

 the grooves, and finally into the vascular, nephridial sac ready 

 to be expelled. At the same time the blood cells and unripe 

 eggs were rejected and thrown back into the ccelomic fluid. 

 The sight was indeed astonishing to see the general precision 

 of a comparatively simple process. 



It was by means of living dissections that the observations 

 were made possible. First it is necessary to use only ripe females, 

 and to determine this fact with certainty one needed to wait 

 until the worm began to deposit eggs; then it was lifted out of 

 sea-water and the surface of the body quickly dried. Next the 

 ccelome was opened and its contents carefully drained into a 

 clean watch glass; this was kept covered to prevent unnecessary 

 evaporation while the nephridia were being removed. The 

 worm was then placed in a dry dissecting pan and pinned at 

 either end, so that it was held in a slightly stretched condition. 

 The body cavity was opened by making a cut through the 

 body w r all in the mid-dorsal line; this cut began a little back of 

 the middle of the body and extended forward to the anterior end. 

 The enteric canal and accompanying blood vessels were next 

 divided midway and their anterior portions removed. The 

 flaps of the body wall on either side were then pinned out. In 

 order to have these flaps spread out well, and so expose the large 



