1a ORGANIC BEHAVIOUR 
the biological end. Such preparatory changes for a future 
contingency are especially characteristic of organic behaviour. 
There is nothing like it in the mineral kingdom. The nucleus 
divides into two parts, one of which passes out of the ovum 
and is lost. The nucleus again divides, and again one part passes 
out and is lost. Thus only one quarter of the original amount 
of nuclear matter remains. Now, division of the nucleus occurs 
whenever an animal cell divides ; but in this case (apart from 
details whieh would here be out of place) there is this difference. 
During the ordinary division of cells there are found in the 
nucleus a definite number of curved rods, and this number is 
constant for any given species; but in the nucleus which re- 
mains in the ovum after three parts of its substance are lost, 
the number of rods has been reduced to half that which is 
common to the species. ‘The egg is now ready for fertilization. 
A minute active cell, which is produced by the male, and 
which also has only half the normal number of rods, enters 
the ovum. The two nuclei approach each other, and give rise 
to the single nucleus of the fertilized ovum, which thus has 
the full number of rods—half of them derived from one 
parent, half from the other parent. The sperm cell of the 
male adds little to the store of protoplasm in the ovum ; but 
it introduces a minute body, which seems to initiate sub- 
sequent divisions of the cell. The nature of these divisions 
may be seen in the accompanying diagrammatic figure. In A 
the cell is just preparing to divide. Above the nucleus is the 
minute body (centrosome) just spoken of, which has already 
divided. In the nucleus the matter of which the rods will 
be constituted is net-like. In this net-work has taken on 
the new form of a coiled thread, while the divided body above 
is associated with a spindle of delicate fibres. In o the 
membrane round the nucleus has disappeared, and the coiled 
thread has broken up into curved rods (chromosomes), four 
of which are shown. The two halves of the minute body 
form the centres of radiating stars. In D each curved rod 
has split along its length, and the two parts are being drawn 
asunder towards the centres of the two stars; the cell itself is 
