26s 



THE AMERICAN NATURALIST 



[Vol. L 



all improbable that such reducing processes are more or 

 less incomplete, so that the protoplasmic substratum in 

 the second generation is different from that of the first. 

 We have evidence of this in the foraminifera where differ- 

 ences in the protoplasmic structure and in shell structure 

 characterize the second generation. Further evidence is 

 seen in the rhizopods, where increasing quantities of chro- 

 midia, and in some cases differences in shell structure, are 

 morphological indications of differentiation. 



Furthermore, it is not improbable that such differences 

 are cumulative from generation to generation, just as 

 chemical differentiation is cumulative with inter-divisional 

 age, until a protoplasmic substratum is evolved in which 

 processes not possible before can now take place. We have 

 shown that Paramecium at the conjugation phase has a 

 different physical make-up than at other times, the cortical 

 plasm becomes mucilaginous and fusion results on contact, 

 while physiological differences are manifested by the in- 

 variably decreasing division rate during and after this 

 period when conjugation is possible. Her© the proto- 

 plasmic substratum is differentiated, and processes occur 

 which are not possible at other times. So, too, in Di- 

 dinium, Stylonychia, etc., with successive generations a 

 protoplasmic substratum is gradually evolved (possibly 

 hastened by adverse conditions) in which the peripheral 

 zone of protoplasm undergoes cytolysis and forms an im- 

 pervious membrane— the cyst membrane— analogous to 

 the fertilization membranes of metazoan eggs. Further 

 cytolytic changes, involving hydrolysis, reduction and 

 other chemical activities, are set up in the cell body, espe- 

 cially in the cell nuclei which divide or fragment. As a 

 result of these activities, which are more profound than 

 those accompanying cell division, the protoplasm is again 

 restored to a labile condition, vitality is renewed and a 

 de-differentiated protoplasm begins a new cycle of meta- 

 bolic and reproductive phases. 



The phenomena of conjugation may be interpreted in a 

 similar way as due to processes possible only in a sub- 

 stratum produced by cumulative protoplasmic differentia- 



