THE BEARERS OF THE HERITAGE 303 
which keeps the chromosomes of constant size, shape and number; 
which partitions them so accurately in ordinary cell-division; 
and which provides for a reduction of their numbers by half in the 
germ-cell while yet securing that each mature gamete gets one of each 
kind of chromosome. Most biologists look on these facts <s indicating 
that the chromosomes are specifically concerned in inheritance. 
In the first place it is recognized that as rezards the definable 
characters which separate individuals of the same species, offspring 
may inherit equally from either parent. And it is a very significant 
fact that while the ovum and spermatozoén are very unequal in size 
themselves, the chromosomes of .he two germ-cells are of the same 
size and number. This parity in chromosomal contribution points 
clearly to the means by which an equal number of character deter- 
miners might be conveyed from each parent. Moreover it i; mainly 
the nucleus of the sperm-cell in some organisms which enters the egg, 
hence the determiners from the male line must exist wholly or largely 
somewhere in the nucleus. And the bulk of the nucleus in the sper- 
matozoén consists of the chromosomes or their products. 
A single set of chromosomes derived from one parent only is 
sufficient for the production of a complete organism.—That a single 
or haploid set of chromosomes as seen in the gametes is sufficient 
contribution of chromatin for the production of a complete organism 
is proved by the fact that the unfertilized eggs of various animals 
(many echinoderms, worms, mollusks, and even the frog) may be 
artificially stimulated to development without uniting at all with a 
spermatozoédn. The resulting individual is normal in every respect 
except that instead of the usual diploid number it has only the single 
or haploid number of chromosomes. Its inheritance of course is 
wholly of maternal origin. The converse experiment in echinoderms 
in which a nucleus of male origin (that is, a spermatozoén) has been 
introduced into an egg from which the original nucleus has been 
removed shows that the single set of chromosomes carried by the male 
gamete is also sufficient to codperate with the egg-cytoplasm in 
developing a complete individual. 
The duality of the body and the singleness of the germ.—Since 
every maternal chromosome in the ordinary cell has an equivalent 
mate derived from the male parent, it follows therefore, supposing the 
chromosomes do have the significance in inheritance attributed to 
them, that as regards the measurable inheritable differences between 
two individuals, the ordinary organism produced through the union 
