176 FOOT-NOTES TO EVOLUTION. 
chromosomes from the egg nucleus and half from the 
sperm, thus containing equal amounts from each par- 
ent. The centrosome, which, as we have seen, is to be 
regarded as the dynamic centre of the cell division, 
comes from the spermatozoon alone; the egg, on the 
other hand, furnishes the yolk and practically all of the 
cytoplasm. 
After this preliminary outline of the facts of fertiliza- 
tion we are in a better position to understand the details 
of a process which occurs in the develop- 
ment of both egg and sperm cells, name- 
ly, the reduction of the chromosomes. 
The necessity for such a reduction is 
evident from a moment’s reflection. We have seen that 
the number of chromosomes in the nucleus is a con- 
stant and typical one for each animal and plant species 
so far as known. As fertilization consists in the union 
of two cells into one, from which the young organism 
develops, it is plain that, were there no reduction, 
the number of chromosomes would be doubled in each 
succeeding generation. However simple this necessity 
for reduction may appear, the minutiz of the processes 
through which it is brought about, and the theoretical 
significance of these facts, form the most involved prob- 
lem of biology to-day. Ina few forms, especially among 
the lower Crustacea, the facts of the reduction are clear 
and relatively simple; in other forms they thus far stand 
in direct contradiction, and, for the present, a compre- 
hensive explanation applicable to all forms must be left 
to further investigation. 
The significance of reduction turns upon the concep- 
tion of a definite organization and individuality in the 
chromosomes and the assumption that they represent 
the physical basis of heredity—i. e., that they influence 
and determine into what the fertilized egg shall develop. 
The reduction 
of the chromo- 
somes, 
