ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 4] 
difference in the staining reactions of the two. Except for the presence of these 
_ astral systems, this cleavage spindle is almost exactly like a maturation spindle. 
In Cynthza 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 Czoza 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 cleavage spindles of Czoza, like the maturation spindles already described, 
are of especial interest for the study of the mechanics of mitosis. I 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 with the centrosomes, but remain at the 
border of the aster, where they are transformed into chromosomal vesicles. Іп the 
maturation divisions there are no centrosomes or asters at all to complicate the 
problem, and here 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. 62, 64). 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. 
The 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 
this 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 oóplasm with exact equality, 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 JOURN. А. N. S. PHILA., VOL. ХПІ. 
