62 REPRODUCTION AND LIFE HISTORY. 
somes undergoes reduction to half the normal. In eps 
cases this reduction comes about through a “ heterotypic ” 
meiotic division. We give a condensed account of what 
happens in a large number of cases. 
The germ-cells grow relatively large ; the nuclear material takes the 
form of a definite number of chromatin loops; at a certain stage it is 
seen that the number is half what it was in more immature stages of the 
germ-cells, and half what it is in the somatic cells of the species under 
consideration. If the normal number be z, it is reduced to 4. There 
has been 2207 of chromosomes. 
The chromatin-loops contract away from the nuclear membrane 
(synapsis) ; the chromatin granules divide so that each loop appears 
doubly-beaded ; the ends of each loop are separated, and there are now 
n bodies with chromatin, each equivalent to a chromosome. 
The ends of the loops move apart, and, with or without a second 
synapsis, they change in shape, unite end to end, and form 2 twin- 
bodies or gemini, sometimes rod-like, sometimes like two brackets, 
sometimes like four dots. 
The nuclear membrane disappears, the = meiotic gemini are set free, 
they become arranged on the division-spindle at right angles to the 
equatorial plane (not flat as in ordinary karyokinesis), with their axes 
parallel to the axis of the spindle. They halve as if transversely, 
separating into two parts which go to the two poles of the spindle. 
Thus each daughter-cell has 7 chromosomes. 
In the case of the ovum the meiotic division usually occurs in the 
formation of the first polar body, so that it and the reduced nucleus of 
the ovum have each 7 chromosomes. There is no further reduction in 
the formation of the second polar body, which involves an ordinary 
equation-division. The first polar body often divides into two. Thus 
the result is one viable cell (the mature ovum) and three non-viable 
cells (the polar Lodies), each with 2 = chromosomes. 
In the spermatogenesis or production of spermatozoa the meiotic 
division is usually the second-last. A ‘“‘mother-sperm cell” or 
spermatogonium divides into spermatocytes with z chromosomes, each 
of these divides into 2 spermatocytes with = chromosomes, and these 
again divide into spermatocytes which differentiate into spermatozoa. 
The result is that from each of the penultimate generation of spermato- 
cytes there arise four spermatozoa, each with = chromosomes. Thus 
there is a close parallelism in the maturation process in the two sexes, 
That the fertilisation of the ovum restores the number to the normal 
is obvious. 
Part of the significance of the long circuitous process of meiotic 
division is that it affords opportunity for fresh permutations and 
combinations of hereditary qualities, for it seems probable that the 
chromosomes are the bearers of these.. 
