THE ORGANIC CELL 
28 
raw beef-steak per day. It is perfectly tame, and wanders all 
over the grounds and through the house. 
The chief causes of death amongst aviary birds are 
pneumonia, avian tuberculosis, constipation through over- 
eating, and anaemia. If any bird shows signs of either of the 
former it should at once be removed. 
THE ORGANIC CELL 
Part IV. — Its Methods of Division and Status in the 
Process of Heredity 
By E. Wynstone-Waters, F.R.S.Edin., &c., Late Senior 
Demonstrator of Anatomy at the Royal College of Surgeons , 
Edinburgh. 
Mitotic cell- division ensures the continuity of life, and 
maintenance of the species, by passing on from cell to cell 
a counterpart of the chromatin which was the determining 
factor in its own organisation. 
Cell- division runs in cycles, with a continual loss of energy. 
Rejuvenescence only occurs after the addition of material 
derived from the nucleus of another cell. The operation which 
results in this admixture is called fertilisation, and is the essen- 
tial factor of sexual reproduction. The result of the fresh 
admixture of nuclear material is twofold : — the energy of 
cell-division is restored, and two separate lines of descent 
become fused in one. The actual reason why this double 
process should take place is unknown. One school of thinkers, 
represented by Herbert Spencer and Hertwig, believe that 
protoplasm shows a strong tendency to pass into a state of 
very stable equilibrium, and that in order to render it more 
responsive the addition of fresh nuclear material is necessary. 
It has been pointed out that the life-history of the Metazoan 
is a parallel to that of the Protozoan, for in both of them, after 
a series of cell-divisions, a period of senescence sets in, which 
can only be prevented by conjugation. After conjugation 
