THE PHYSICAL BASIS OF HEREDITY. 163 
according to Henneguy. CAilodon is a minute fresh- 
water infusorian, which multiplies for a considerable 
period of time by transverse division. After a time, 
however, the physiological necessity for conjugation 
ensues. The animals having placed themselves side by 
side in pairs and partly fused together, the nucleus of 
each individual divides into two portions, one of which 
passes from each infusor into the other to unite with the 
half remaining stationary. The two then separate, each 
having received a half of the nucleus of the other. 
After thus trading experiences, as it might be termed, a 
period of renewed vigour and activity for each sets in, 
manifested in rapid growth and multiplication by divis- 
ion, producing a large number of generations, which 
continues until weakening vital activities indicate the 
periodically recurring necessity for conjugation. In gen- 
eral, among the Infusoria we find the same process tak- 
ing place in regular cyclical order, with more or less 
complicated variations of the phenomena just outlined 
for Chilodon. In all of them the aim of the conjugation 
is the same, the exchange of a certain amount of nuclear 
substance between the two conjugating individuals, and the 
same physiological effect is reached, a rejuvenescence, 
as it were, of the two organisms which manifests itself 
in renewed vigour of growth and multiplication. 
In some of the lowest forms of unicellular life—for 
example, the Schizomycetes or bacteria and their allies 
—this necessity for conjugation does not appear to 
exist, but for the vast majority of forms this cyclical 
law of development holds good. In the Protozoa no 
division into somatic and germinal cells is found, both 
functions being united in the one cell which forms the 
whole body of the organism. .In the Metazoa, however, 
this differentiation has taken place; the germinal cells 
are set apart for the preservation of the race; the so- 
