264 CONK LIN. [VOL. II. 



chromosomes in each spindle, the same number that is found 

 in each germ nucleus. 



4. In each of the germ nuclei, before they come into contact, 

 there is a single nucleolus ; these nucleoli disappear in the pro- 

 phase of the first cleavage, but in the succeeding telophase a 

 single nucleolus generally appears in each half of each daughter- 

 nucleus. The same is true of the succeeding cleavages, so that 

 each nucleus throughout the cleavage usually has two nucleoli 

 in the telophase or early resting stage, though the number may 

 vary in the later resting period, as pointed out above. The 

 fact that there is a single nucleolus in each germ nucleus, and 

 that there is usually a single nucleolus in- each half of the 

 double nuclei of the cleavage, may possibly indicate that these 

 halves are each derived from one of the germ nuclei. Since 

 the nucleoli as such do not persist throughout the mitosis, may 

 it not be possible that there is some achromatic structure in 

 connection with them which does persist and form the basis 

 for the new nucleoli which appear in the daughter-nuclei ? 



These facts make it very probable that the germ nuclei of 

 Crepidula preserve their individuality throughout the cleavage, 

 though their separateness may be apparent only or chiefly in a 

 single stage of the nuclear cycle, viz., the telophase. Further, it 

 is possible, even in an advanced stage of the cleavage, to deter- 

 mine with considerable probability which part of a double 

 nucleus is derived from the egg and which from the sperm, 

 the egg half always lying nearer the animal pole than the 

 sperm half. Finally the initial position of the mitotic spindle 

 seems to be determined by the relative positions of the halves 

 of the double nuclei, since the spindles when they first appear 

 lie in the plane of contact between the two halves ; the final 

 position of the spindle and the direction of division are deter- 

 mined by the movements of the cytoplasm. 



