irs of chromosomes, but even that it is possible to
locate one of these groups in a particular chromosome and to state the
_relative position_ there of the factors for the characters. If the
validity of this evidence is accepted, the study of the cell leads us
finally in a mechanical, but not in a chemical sense, to the ultimate units
about which the whole process of the transmission of the hereditary factors
centers.

But before plunging into this somewhat technical matter (that is difficult
only because it is unfamiliar), certain facts which are familiar for the
most part should be recalled, because on these turns the whole of the
subsequent story.

[Illustration: FIG. 47. An egg, and the division of the egg--the so-called
process of cleavage. (After Selenka.)]

The thousands of cells that make up the cell-state that we call an animal
or plant come from the fertilized egg. An hour or two after fertilization
the egg divides into two cells (fig. 47). Then each half divides again.
Each quarter next divides. The process continues until a large number of
cells is formed and out of these organs mould themselves.

[Illustration: FIG. 48. Section of the egg of the beetle, Calligrapha,
showing the pigment at one end where the germ cells will later develop as
shown in the other two figures. (After Hegner.)]

At every division of the cell the chromosomes also divide. Half of these
have come from the mother, half from the father. Every cell contains,
therefore, the sum total of all the chromosomes, and if these are the
bearers of the hereditary qualities, every cell in the body, whatever its
function, has a common inheritance.

At an early stage in the development of the animal certain cells are set
apart to form the organs of reproduction. In some animals these cells can
be identified early in the cleavage (fig. 48).

The reproductive cells are at first like all the other cells in the body in
that they contain a full complement of chromosomes, half paternal and half
maternal in origin (fig. 49). They divide as do the other cells of the body
for a long time (fig. 49, upper row). At each division each chromosome
splits lengthwise and its halves migrate to opposite poles of the spindle
(fig. 49 c).

But there comes a time when a new process appears in the germ cells (fig 49
e-h). It is essentially the same in the egg and in the sperm cells. The
discovery of this process we owe to the laborious researches of many
workers in