THE PRESENT POSITION OF CELL-THEORY. 313 



Within the limits of the protozoa we study many kinds 

 of bionts which, whilst retaining great simplicity of structure, 

 have advanced far beyond the stage represented by these 

 simple forms. 



The most important as well as the most striking 

 structural advance is the formation of a nucleus. The 

 nuclein which was, in the simplest bionts, distributed 

 through the protoplasm, is aggregated to form a compact 

 body, which from its structure and behaviour may be re- 

 garded as a metabiont, as also may the part from which it 

 was segregated, the cytoplasm. The steps which lead up 

 to the segregation of the nucleus are obscure, but there are 

 very good grounds for saying that the nucleus, when formed, 

 is connected, in some manner unknown to us, with the 

 transmission of the so-called historic qualities of the biont. 

 In any case it plays a leading part in reproduction, and the 

 steps from the condition of diffused nuclein to centralised 

 nuclein are suggested by the infusorian Holosticha scutelhim, 

 which ordinarily has no definite nucleus, but contains 

 numerous chromatin particles scattered throughout its sub- 

 stance. Previous to reproduction by division the scattered 

 particles are drawn together and unite to form a centralised 

 nucleus, which divides in a normal manner and breaks up 

 again into particles in the offspring. 1 



Besides the nucleus many other structural advances are 

 to be noted in protozoa and in unicellular plants ; some 

 must be regarded as metabionts, e.g., chlorophyll corpuscles 

 and chromatophores of various kinds, many kinds of granules, 

 etc. Other structures cannot be regarded as belonging to 

 the same category, e.g., cilia, contractile fibres, etc. We 

 may for the present purpose leave both cases out of con- 

 sideration, for it is the nucleus and the part it plays as an 

 essential constituent of the biont which most concerns us. 



We have as yet very obscure notions about the co-opera- 

 tion of nucleus and cytoplasm in the production of vital 

 phenomena. But, putting aside the views of those who 

 postulate the existence of minute vital units, and speak of 



1 Aug. Gruber, " Ueber vielkernige Protozoa," Biol. Centralblatt, iv., p. 

 170. See also the same author, Zeit.fiir Wiss. Zool., xli., p. 186. 



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