THE BIOLOGY OF THE CELL SURFACE 



cleavage or zygote nucleus about which form two asters of 

 unequal size. The larger of these which can be traced 

 continuously is derived from the larger sperm-aster; 

 the smaller, it is believed, though it can not be followed 

 continuously, represents the smaller sperm-aster. Thus 

 in the egg of Nereis both cleavage-asters are derived from 

 the sperm-asters, if it be true that the smaller sperm- 

 centrosome and -aster persist. Since the middle-piece 

 remains outside of the €:gg, the cleavage-centres held to be 

 genetically continuous with those of the sperm-nucleus 

 can not be derived from the middle-piece. 



The history of fertilization in the egg of another marine 

 worm, Chaetopterus pergamentaceus, as given by Mead,^ 

 may be taken to represent eggs of Class 2 which reach the 

 fertilizable stage after break-down of the germinal vesicle. 



These worms inhabit U-shaped tubes thirty to forty 

 centimeters long, only the ends of which project above the 

 mud. In the laboratory, these bizarre-looking animals, 

 having been removed from their tubes, may be kept for 

 some days in running sea-water. The sexes are readily 

 distinguished: the eggs give the posterior segments of the 

 female a pale yellow color, whilst the similar segments in 

 the male which contain spermatozoa appear milky-white. 

 It is best to use eggs and spermatozoa removed from ani- 

 mals kept each in a separate container of gently flowing 

 sea-water within twenty-four hours after the animals have 

 been collected. 



When laid or removed from the animal, the eggs of 

 Chaetopterus are in the germinal vesicle stage, but when they 

 come into the sea-water the germinal vesicle breaks down, 

 and the first maturation spindle forms near the centre of 

 the egg. This moves to the animal pole of the egg and in 

 this stage the egg remains until death unless fertilized or 



1 Mead, 189S. 



164. 



