THE CHROMOSOMES 143 
Haans, Herbst, and Boveri on giant eggs of sea 
urchins fertilized by sperm of another species. The 
hybrid larve produced when normal eggs of one 
species are fertilized by sperm of the other species 
are intermediate in character between the two 
parental types of larvee; while those from giant eggs 
of the same species fertilized by sperm of the other, 
also intermediate, incline more to the maternal side. 
The nucleus of the giant egg is double the size of 
that of the normal egg and according to Bierens de 
Haans the chromosomes are also double in number. 
Consequently, the amount of maternal chromatin 
should be double that introduced by the sperm, and 
might produce a corresponding influence: on the 
hybrid character. But since in these giant eggs the 
cytoplasm is also doubled, it is not evident that the 
results are due to the chromosomes rather than to the 
cytoplasm. By means of the following ingenious 
comparison Boveri has shown that the results must 
be ascribed to the chromosomes rather than to 
cytoplasm. Normal eggs were broken into frag- 
ments, the nucleated pieces were fertilized with the 
sperm of the other species, and those fragments of 
half the volume of the normal egg were isolated. 
As is known, such fragments develop into whole 
larvee, whose nuclei will have the usual chromatin 
content. The egg cytoplasm is, however, reduced to 
half. Nevertheless the larve did not incline to the 
paternal side, although these larvae, like all larve 
from fragments, were often simpler than the normal. 
Hence since a relative decrease in the amount of 
