GENERAL CONSIDERATIONS 



17 



B, THE RECIPROCAL RELATIONS BETWEEN THE NUCLEUS 

 AND PROTOPLASM 



Most animal and plant cells are nucleated. Only in the Bacteria is the 

 presence of a nucleus doubtful. Some authors claim indeed that these organ- 

 isms also are to be added to the general cell scheme; but the facts which 

 have been brought forward in support of this view are far from sufficient 

 to constitute actual proof. Although the red Hood corpuscles of the Mam- 

 malia contain a nucleus at an early stage of their development,, in their mature 



FIG. 14. Polystomella venusta, a microscopic organism surrounded by a calcareous shell, after 



Max Schultze. 



state they are, so far as we know, without nuclei. If so, they are scarcely to 

 be regarded as cells, since having lost the nucleus they are no longer capable 

 of reproduction. 



Wherever it occurs the 'nucleus represents a necessary constituent of the 

 cell. Single-celled forms may be divided by a sharp cut or by other means 

 into a nucleated and a nonnucleated part. The former is soon regenerated 

 to a complete cell even if it contain but a portion of the nucleus; while the 

 nonnucleated part invariably dies after a short time, for although it may 

 move about quite normally, may ingest foreign bodies (Infusoria), and even 

 kill them, no digestion, or at best only a partial digestion, can take place 

 (Nussbaum). 



The production of certain substances on the part of the protoplasm is 

 likewise stopped by the removal of the nucleus. A nonnucleated portion of 

 Polystomella (Fig. 14) is no longer able to elaborate calcium carbonate, while 

 a nucleated piece at once makes good any defect in its calcareous shell by the 

 deposit of new carbonate at the injured place (Verworn). In plants it has 

 been observed that an isolated piece of protoplasm is unable to construct a 

 new cellulose membrane (Klebs). 



