272 Causes and Course of Organic Evolution 



alveolar sac, with chromatin masses irregularly scattered or 

 massed toward the interior of the nuclear membrane {Pelomyxa^ 

 Lithamoeba). Or the protoplasmic substance may be inter- 

 penetrated by a fine chromatin network that we would regard 

 as homologous with the kinoplasm of the plant cell, and as 

 radiating from the nuclear membrane, while the nucleus shows 

 a more or less granular chromatin texture (Difflugia). Again, 

 as in the plant cell with its plastids, some of these chromidial 

 masses may be specially set aside for nutritive purposes, when 

 they have been called trophochromidia, or they may represent 

 detached parts of the nuclei that are specially concerned in 

 irritable and reproductive action, when they have been called 

 idiochromidia (Centropyxis). Or again a paranuclear mass of 

 chromatin lies near or beside the nuclear membrane, and seems 

 to take active part in conjugation as in Paramoeha. Finally 

 a distinct nucleolus or chromatin center to the cell may be 

 formed as in Arcella. 



In the fresh-water Foraminifera or Gromiadse there is usually 

 a well-defined nucleus, and this is also true of the Sporozoa, 

 where the nucleus with a distinct nuclear membrane and en- 

 closed nucleolus or nucleoli are evident. 



From the above it will be seen that the amount and degree 

 of differentiation of the chromatin varies from a condition 

 where it exists as scattered chromidial granules in the proto- 

 plasm, through stages of evolution to where a complex chro- 

 matin network traverses the protoplasm, passes inward to the 

 nuclear membrane, is continued inward as a nuclear network, 

 and finally becomes centered in the nucleolus or nucleoli. 



The second and more complex group shows even greater 

 and more varied chromatin relations than the first. The 

 striking difference between megalospheric marine Foraminifera 

 with their single large nucleus, and microspheric forms with 

 many small nuclei, is still so little understood in its exact bio- 

 logical significance that it would be superfluous to treat of 

 such here. Amongst the Radiolaria there is a great diversity 

 of detail. But in not a few, such as the Acantharia and Dis- 

 coidea, fine threads from the nucleus radiate outward through 



