APPENDIX. 
125 
Somewhere in the interior of the cell, and generally about 
the middle, can be seen by the microscope a small round spot, 
generally of a slightly darker colour than the rest of the cell — this 
is the nucleus. When the nucleus is examined with the highest 
magnifying powers, it is found to be an extremely complex body ; 
it is separated off from the rest of the cell by an extremely delicate 
membrane or wall, which, however, under certain circumstances, 
sometimes disappears. It is permeated by an extremely fine and 
complicated network of threads. When a cell is about to divide, 
some curious changes take place in the nucleus, and particularly 
in the network of threads which I have mentioned. The threads 
cease to be a network, and assume the form of a single coiled 
fibre ; next, this break's up into a number of detached pieces, 
which then arrange themselves in the form of a series of loops 
around the equator of the cell. Each of these loops then split 
along its whole length as if a piece of untwisted rope had its 
strands separated into two thinner ropes, and the split portions 
are drawn gradually towards the two ends of the cell, where they 
join themselves together into two bundles, are followed by the 
rest of the nuclear substance, and form two separate nuclei. 
While this process has been going on the protoplasm of the cell 
has been tending to collect it: e'.f around the two daughter nuclei, 
and by the time the nucleus has fully divided, the rest of the 
protoplasm is ready to follow its example and to divide into two 
parts. This complicated process takes place whenever a cell 
divides ; that is to say, it is taking place in millions of cells in 
almost every growing plant. To it the name of mitosis has been 
given by biologists, and it is highly suggestive of the extreme 
importance of the nuclear threads of which I have spoken, and to 
which the name of chromosomes has been given, because they 
have the property, in a pronounced degree, of absorbing colouring 
matter, which may be brought in contact with the cell to which 
they belong. Now an egg cell is developed from the ovary in the 
same way that other cells are developed. When, however, an 
egg cell is fully developed or ripe, its nucleus divides into two 
parts, but, instead of the whole cell dividing, one of the halves of 
the nucleus is simply thrust outside the cell, and there perishes. 
The part thrown out is called a polar body. The nuclear loops, 
however, in this case are not split, but simply one-half of their 
number are removed in their entirety. (See first diagram.) From 
that moment no further development takes place in the egg cell 
until fertilisation has occurred, and that process consists in the 
replacement of the lost half of the nucleus by another half nucleus, 
containing a similar series of rods or loops, from a sperm cell. All 
this, of course, is a matter of observation and fact, and not of 
theory. 
