110 
THE ORGANIC CELL 
followed by that of the cytoplasm, the process being usually of 
the mitotic type. This primitive mode of reproduction 
continues in many forms for a number of generations, then 
comes the demand for a fusion with another individual, i.e. 
conjugation. Should any external influence prevent such a 
union, the animal shows rapid signs of degeneration which is 
followed by death. This weakening and loss of the functions 
of growth and reproduction is doubtless due to the exhaustion 
of the nuclear substance, for, if fresh nuclear material from a 
different individual be introduced, all the symptoms of senile 
decay disappear, and there is a rejuvenescence of the whole form. 
This phenomenon is beautifully illustrated in a fresh-water 
infusorian called Chilodon. This form multiplies for some 
time by the simple process of transverse division. Exhaustion 
at last supervenes, and the necessity for conjugation occurs. 
The animals arrange themselves side by side, and the nucleus 
of each divides into two, one half remains stationary, the 
other half migrates to the adjacent infusor and unites with the 
stationary half. The two forms then separate, having received, 
each of them, a half nucleus from the other. After this exchange 
of courtesies, the two forms lead the usual solitary existence. 
It is very noticeable, however, that they are now charged with 
fresh energy, and the various phenomena of growth, repro- 
duction, &c., are carried out with great vigour. This continues 
for a considerable period, until the waning energies warn the 
individual of the necessity for a fresh conjugation. 
No one can doubt that this necessity for conjugation is 
the demand for a fresh supply of nuclear substance (chromatin) 
from another individual, and in all cases where the exchange 
has been accomplished the results are the same — a complete 
rejuvenescence of all the animal functions. Neglect to con- 
jugate results in certain death. 
In bacteria and their allies the process of conjugation does 
not occur, but in the great majority of simple forms the cyclical 
phenomena above described maintain. As we have been able 
to observe, in the protozoan there is no separation of cells into 
somatic and germinal, the organism itself consisting of a 
single cell, and the functions of body formation and cleavage 
to form a fresh generation are inherent in the one mass. 
