THE PHYSICAL BASIS OF HEREDITY. 163 



according to Henneguy. Chilodon 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 fiuclear 

 substance betiveen 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- 



