374 ^-^^ AMERICAN NATURALIST. [Vol. XXXVIII. 



in its presence throughout all periods in every cell's history. 

 It passes on from cell to cell through the mechanism of nuclear 

 division without interruption. There are periods of cell history 

 when the nucleus consists only of chromosomes as in the stages 

 of nuclear division called mataphase and anaphase. The other 

 structures of the nucleus have their relation to definite condi- 

 tions that are in part understood. The nuclear membrane 

 probably results from the reaction of the cytoplasm to the secre- 

 tion of nuclear sap among the chromosomes (Lawson, :03 a). 

 It would then be strictly cytoplasmic in character and similar to 

 the plasma membranes around vacuoles. Nucleoli must be 

 regarded as temporary structures since they generally disappear 

 during nuclear division either dissolving or else passing out into 

 the cytoplasm where they may remain for long periods as deeply 

 staining globules (extra nuclear nucleoli). Linin is believed to 

 be derived from chromatin and in its turn may be transformed 

 into the substance 6i spindle fibers, which are cytoplasmic, so 

 that chemically it holds a position somewhat intermediate between 

 chromatin and cytoplasm. It seems established that the linin 

 net work is a temporary structure related to the activities of 

 chromatin. 



(b) The Plastids. 



These very interesting structures, characteristic of plant cells, 

 have not received the degree of attention that they deserve 

 and much valuable work may be done in the detailed study of 

 their protoplasmic structure and activities at various periods of 

 ontogeny especially through the series of changes that are 

 presented during developmental processes. 



The primitive types of plastids are relatively large structures, 

 often solitary in the cells, and generally of complex form. 

 These are called chromatophores and are characteristic of many 

 algas especially among the lower groups but are not found above 

 the thallophytes (Anthoceros and Selaginella excepted). 



The chromatophores of the simplest algse are replaced in most 

 of the higher types of these thallophytes and in all groups 

 above by very much smaller structures, generally discoid in 



