IOWA ACADEMY OF SCIENCES. 
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46 
periods of cell activity; the dependence of the cytoplasm 
upon the nucleus in regeneration experiments; and the 
real value of the nucleus in the field of heredity. This 
does not mean that the nucleus is more important than the 
cytoplasm in cell-life, for the two stand in a mutual 
relationship necessary to each other. 
The importance of lecithin in metabolism has been 
shown by the feeding experiments of Hatai. This sub- 
stance evidently represents a distinct step in the process 
of assimilation in the animal cell. 
The absence of carbohydrates and fats from the nucleus 
signifies that these substances are transformed and split 
up entirely in the cytoplasm. The place of proteid in 
metabolism, then, may be regarded as primary; that of the 
carbohydrates and fats as secondary. 
In the field of excretion there is less of experimental 
evidence to serve as the basis for an adequate conception, 
and it is probable that quite diverse mechanisms must be 
recognized. The work of Dreser is very suggestive, how- 
ever, showing that excretion from the cells of the urini- 
ferious tubules may be calculated on the basis of osmotic 
pressure. 
IV. THE TRANSFORMATION OF ENERGY IN THE CELL. 
Animal biology may begin to claim recognition as an 
exact science. It has passed beyond the merely descriptive 
stage and has demonstrated beyond question that the 
principle of the conservation of energy really holds in the 
activity of the animal cell. Potential energy is introduced 
into the cell from without along with the matter of food 
and oxygen. Foods are substances whose elements have 
a relatively feeble affinity for each other as compared 
with the affinity which each element has for oxygen. 'The 
accurate measurement of the potential energy of foods is 
necessarily recent; the determination of the kinetic energy 
of the organism, even more so. But the difficulties have 
been overcome by the intelligent co-operation of many 
workers: — the efforts of Atwater and his collaborators 
should be especially mentioned. We stand today on a 
