CHAPTER V

■^HE CHROMOSOMES AS BEARERS OF
HEREDITARY MATERIAL

The evidence in favor of the view that the chromo-
somes are the bearers of hereditary factors comes from
several sources and has continually grown stronger,
while a number of alleged facts, that seemed opposed
to this evidence, have either been disproven, or else
their value has been seriously questioned. We pro-
pose now to examine in some detail the observations
and experiments that bear on the chromosome theory
of heredity.

The Evidence from Embryology

Relating   to   the   Influence   of  the   Chromosomes   on
Development

It has been argued that since the sperm transmits
equally with the egg, and since only the sperm. head,
consisting of the nucleus, enters the egg, inherit-
ance is only through the nucleus. But it must be
admitted that around the entering sperm nucleus
there may be a thin enveloping protoplasm, which,
however scanty, might suífice to transmit certain
cytoplasmic factors. Moreover, while the tail of
the sperm appears in some cases to be lef t outside the

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