ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. II 



difference in the staining reactions of the two. Except for the presence of these 

 a>tral systems, this cleavage spindle is almost exactly like a maturation spindle. 



In Cynthia the distinction between these two parts of the cleavage spindle is 

 not so clear because here the nuclear portion is much Longer and reaches nearly to 

 the centrosomes (fig. 97); but even here the nuclear part can be distinguished from 

 the astral by the stronger character of its fibres. In this genus also the astral sys- 

 tems are not so Large as in Ciona and the individual fibres are stronger, so that the 

 contrast between the nuclear and the astral portions of the spindle are but faintly 

 marked. 



These clea\ age spindles of Ciona, like the maturation spindles already described, 

 are of especial interest for the study of the mechanics of mitosis. 1 have not attempted 

 to make a detailed study of this subject, but it is quite evident that the separating 

 chromosomes move only as far as the ends of the nuclear spindle fibres (fig. 99). 

 They are never drawn up into contact witli the centrosomes, but remain at the 

 border of the aster, where they are transformed into chromosomal vesicles. In the 

 maturation divisions there are no centrosomes or asters at all to complicate the 

 problem, and liere also the chromosomes move only to the ends of the nuclear spin- 

 dle fibres. These fibres elongate somewhat in the later stages of mitosis (figs. 70, 

 71, 72), thus separating more widely the daughter chromosomes. 



The fact that in the maturation divisions the chromosomes separate without 

 the aid of centrosomes or asters may be taken as evidence that in the similar spin- 

 dles of the first and second cleavages the centrosomes and asters, although present, 

 take no part in this work. Anything which will explain the movements of the 

 chromosomes in such spindles as those shown in figures 69 to 72 will also explain 

 their movements in such cleavage spindles as those shown in figures 177 and 179. 

 In the maturation mitoses there are neither centrosomes nor asters, and yet the 

 separation of the chromosomes occurs in the usual manner. The spindle fibres 

 apparently serve only as a guide for this movement, and must be considered the 

 result rather than the cause of stresses in the cell substance. This is shown by 

 the fact that when they first appear these spindle fibres are not parallel but run 

 in all directions (figs. 02, G4). Later, under the influence of stresses in the cyto- 

 plasm, they become parallel. Under these circumstances there is no reason to 

 believe that the movement of the chromosomes is caused by other factors than 

 those which bring about movements in the cell body. 



Tile constriction of the cell body first occurs, as indicated by figure 20. at the 

 posterior pole. This is probably due, as Castle says, to the fact that at this stage 

 tliis pole is the more richly protoplasmic one. Very soon the constriction extends 

 all the way around the egg and, as the mitotic spindle lies in the middle, it follows 

 that the constriction must be about as deep on one side as on another (fig. 99). 

 This constriction divides the ooplasm with exact ecpiality, not only quantitatively 

 but qualitatively also. During and immediately following this division the yellow- 

 crescent undergoes some very remarkable transformations. These changes are 

 shown in figures 21 to 26, which represent consecutive stages of the same egg drawn 



6 JOUEN. A. N. S. PHILA., VOL. XIII. 



