IOWA ACADEMY OF SCIENCES. 
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least farthered our understanding of the mechanism in- 
volved. Experimental methods have shown that astral 
rays and spindle fibres are rows of vesicles spun in the 
cytoplasm, expressive of currents or lines of chemical 
action. That much-discussed thing, the centrosome, is no 
longer considered a definite morphological body, but rather 
a center for a special transformation of energy, to be 
induced at points in the cell other than the normal ones 
under certain conditions. The division of the cytoplasm 
certainly involves a localized change of surface tension; 
and the forms assumed by the individual blastomeres 
during cleavage involve surface tension as one of the 
factors. 
The studies of Wilson, Driesch, Morgan, Lillie, Conklin, 
Boveri, Crampton, and others on localization in the egg 
are among the most, important of modern times. Begun 
for the purpose of putting to experimental test the hypoth- 
eses of Roux and Weismann, this line of work has long 
since outgrown its original impetus, and has illumined 
some of the deepest problems of the cell. Differentiation, 
as these studies have shown, is a feature of the cytoplasm. 
It is in the nature of a progressive change, whereby the 
cytoplasm of the egg becomes ever more and more local- 
ized into definitive regions, it may be very early in the 
history of development. Cleavage is merely a means of 
dividing up this material into cell-units; the exact process is 
relatively unimportant, differentiation lying far behind 
cell-boundaries. Localization, while thus cytoplasmic in 
character, is really determined from the nucleus, and is 
carried out in the cytoplasm through specific metabolisms 
which are set up. There is more than a simple parallel 
between the course of determination and the course of 
synthesis in the cell. Chromatin is the center for the 
initial steps of both, and the nucleus holds a definite place 
in development because of the place it holds in metabolism. 
The energy of growth is indeed a conspicuous feature of 
the developing ceil. To say that growth occurs because of 
the introduction of material from without is a mere truism; 
the forcible expansion of the cell is indicative of an energy 
