LIFE HISTORY OF DIDINIUM 229 



Structures of the endoplasm 



Thon has given an excellent account of the finer structures of 

 the endoplasm of Didinium. One or two points should be men- 

 tioned here as they have to do with structures involved in 

 different stages of the life history. The most important of these 

 are the nuclei and their derivatives. 



The macronucleus is correctly described by Thon. In the 

 resting stages it is characterized by deeply-staining spherical 

 granules of chromatin embedded in a more feebly-staining 

 matrix. These bodies in the nucleus behave during division 

 like the chromatin bodies of Dileptus gigas, where they are dis- 

 tributed throughout the cell. At periods of division of Didin- 

 ium they form first a more or less complicated reticulum by 

 elongating and fusing at one or more points. The strings of 

 chromatin are ultimately divided, again as in Dileptus. At 

 periods of encystment the distinct granules of the nucleus be- 

 come much larger and are discharged from the nuclear mass 

 until the cytoplasm becomes filled with densely-staining chro- 

 matin bodies. After recovery from encystment the distributed 

 chromatin masses are broken up into smaller metaplasmic gran- 

 ules, which give a uniformly dense stain to the entire endoplasm. 

 Finally, at conjugation, these metaplasmic bodies disappear from 

 the endoplasm and are concentrated in a deeply-staining cortical 

 armature in the ectoplasm. 



The micro nuclei were entirely overlooked by Thon. They are 

 extremely small and difficult to distinguish from the numerous 

 spherical bodies distributed throughout the endoplasm. At 

 periods of conjugation, however, they are plainly evident and 

 their history may be followed with comparative ease. This was 

 first done by Prandtl ('07), who also for the first time described 

 the micronuclei in vegetative stages. The number, according to 

 Prandtl, is variable, two or three being usually present and 

 these are closely anchored to the macronucleus (fig. 7). In 

 division one pole of the spindle is usually embedded in the sub- 

 stance of the macronucleus. I have confirmed these observa- 

 tions of Prandtl, finding as many as four micronuclei during the 



