THE PHYSICAL BASIS OF HEREDITY. 185 
greatly increase this probability. The offspring resem- 
bles both of its parents, and the paternal tendencies 
can be conveyed in the minute sperma- 
The chromatin tozoan head alone, which is constituted 
as the bearer of : : 
hereditary alle almost entirely of chromatin. The scrup- 
ences: ulous exactitude with which, in both 
germ cells, the chromosomes are reduced 
to one half the normal number preparatory to the union 
of the pronuclei in fertilization, and the distribution of 
the paternal and maternal chromatin 
equally to the resulting cells of cleav- 
age, lend added weight to the theory. It 
remained for the genius of Boveri by a brilliant experi- 
ment to raise this hypothesis to the plane of almost ab- 
solute certainty. The crucial test of the theory would 
be to remove the nucleus from one cell 
and to substitute it in the place of the 
nucleus of another. If the nucleus and 
cytoplasm thus brought into union are so constituted 
that they can exist together, then one of three things 
will happen. Either the qualities of the cell from which 
the nucleus was taken will develop, or those of the 
cytoplasm, or those formed by the union of both nucleus 
and cytoplasm. 
In this first case nuclear control will be demon- 
strated; in the second, cytoplasmic; and in the third, 
an interaction of both nucleus and cytoplasm will deter- 
mine the activities of the cell. 
Such an experiment was first tried by Rauber in en- 
deavouring to remove the nucleus of the fertilized frog’s 
egg, and to substitute for it the nucleus from a toad’s 
egg in the same stage of development. If the nucleus 
contains the hereditary influences, then a toad must 
develop from the union of frog’s cytoplasm and toad 
nucleus. The profound disturbances set up in the 
Indirect evi- 
dence. 
Direct experi- 
mental evidence. 
