270 The Queensland Naturalist. Vol. 1 
For a long time it was believed that the nucleus of the 
cell was destroyed at each division, and the nuclei of the 
daughter cells were entirely new, therefore, as they could not 
carry anything, they could be ignored in any theory of 
heredity. Better methods and more powerful instruments, 
however, revealed the existence of remarkable structures. 
It was found when the reticular frame work of the 
nucleus prepares to divide, it separates into single segments, 
which, becoming thicker and denser, are formed into rods 
of uniform thickness. From the fact that they readily 
accept different stains, they are called '‘chromosomes.” In 
the course of division each chromosome is divided into two 
exactly equal halves. The nuclear membrane then dis- 
appears, and cytoplasm, e.g., the contents of the cell invade 
the nuclear area centreing around special bodies called centro- 
somes. Radiating lines into the cell-plasma suggest that 
these bodies are centres of force. A spindle from each 
pole is formed, the fibrilla of each grasping the longitudinally 
divided chromosomes from the two opposite sides, and arrange 
them as the nuclear plate Each half cliromosome is 
connected with one spindle only, and is then drawn toward 
that pole. The cell then divides into two. Compared with 
the process of dividing the nucleus, that of the cytoplasm is 
very simple, and this led to the conception that the cell 
nucleus is the chief, if not sole, carrier of the hereditary 
characters of the organism, which was held to be proved as 
in flowering plants, only the nucleus of the sperm reaches 
the ovum, and they still show all phenomena of heredity. 
Further investigation showed certain differences in 
reproductive cells — -gonotokonts^ — and body cells, which 
carry fewer chromosomes. A peculiar characteristic of 
gonotokonts was also observed, a very rapid division following 
one another. Further, that the chromosomes unite in pairs. 
It is these pairs, and not the single chromosomes which are 
attached by the fibrillae, so that the number of chromosomes 
contained by the daughter-cells is reduced by half, while 
in the division of ordinary cells the number remains the 
same. Each of the chromosomes has already completed 
the longitudinal fission, but in the next division, which follows 
rapidly on the first, they are not succeeded by an immediate 
separation and allotment to different nuclei, and are therefore 
ready for the next division Through a. number of careful 
observations the fact was established that the doubling of 
the chromosomes, which necessarily accompanies fertilisation, 
is maintained in the product of fertilisation to be again reduced 
to one half in the gonotokonts at the stage of reduction 
division. Thus, the conception of the essence of true 
alternation of generations containing double and single 
chromosomes alternating with one another was formed 
