78 EDWIN G. CONKLIN 
cytoplasm grows at the expense of yolk. In cases in which the 
centrifuging occurred after the spindle was anchored in the cell, 
but before the division wall had formed, the cytoplasm may be 
thrown almost entirely to one pole of ‘the spindle and the yolk 
to the other; accordingly when division occurs one of the daughter 
cells will contain almost all the cytoplasm, the other all the yolk, 
while both cells will receive the same number and mass of chro- 
mosomes, fig. 16. The chromosomes which are left in the yolk 
field remain small and compact since there is no cell substance 
which they can absorb. After some time the yolk in the vicinity 
of the chromosomes may begin to disappear and cytoplasm to 
appear in its place. It can scarcely be doubted that some sub- 
stance, probably an enzyme, is given off by the chromosomes and 
dissolves the yolk, and that this dissolved yolk is then converted 
into cytoplasm through the influence of the chromosomes. Once 
a small field of cytoplasm is formed around the chromosomes, they 
begin to absorb it and to become vesicular. The process of form- 
ing cytoplasm may then go forward rapidly and in the end the 
yolk cell may give rise to protoplasmic micromeres in a normal 
manner (fig. 31). It is probable that a small amount of cyto- 
plasm, which cannot be displaced by centrifuging, is left in the 
yolk cell, and it is possible that the formation of new cytoplasm 
would not take place in the absence of this small remnant, but 
it can be proved conclusively that this formation of cytoplasm 
takes place only in the vicinity of the chromosomes, and that in the 
absence of this chromatic material it never occurs at all. Under 
these circumstances the conclusion seems justified that the chro- 
matin has the power of forming cytoplasm when placed in a suit- 
able nutrient medium, such as yolk, and that the cytoplasm in 
turn contributes to the growth of the nucleus and of the chromatin. 
5. Unequal and differential cell divisions. By centrifuging, the 
size and constitution of the blastomeres may be changed; divi- 
sions which are normally equal may be made unequal, and vice 
versa; cells which are normally protoplasmic may be filled with 
yolk and vice versa. In this way both the cell size and the cell 
content may be controlled experimentally. 
