66 PRINCIPLES OF ANIMAL BIOLOCY 



manner in which protoplasm is irritated and transmits impulses is un- 

 known. From experiments on higher animals it has been ascertained 

 that muscle may be stimulated to contraction after the nerve-connection 

 has been cut and that the impulse may be transmitted from muscle cell to 

 muscle cell. These experiments and others of similar nature show con- 

 clusively that irritability is a general property of protoplasm and not the 

 property of any particular group of cells. 



Relation of the Nucleus to Metabolic Activities. — In the discussion 

 of cell physiology up to this point it has been assumed for the sake of 

 simplicity that all parts of the cell were concerned in the functions dis- 

 cussed, with the possible exception of the chloroplasts in their function 

 of photosynthesis. This assumption is not justified, for the greater 

 number of functions of the cell are very complex, so complex, indeed, as 

 to require the cooperation of several parts of the cell in their perform- 

 ance. In the present state of knowledge it would not be profitable 

 to attempt a complete account of the interaction of the various parts 

 of the cell in the different processes. It is possible, however, to point 

 out one conspicuous phase of this cooperation, namely, the relation of 

 the nucleus to the cytoplasm in certain types of function. It may be 

 stated in advance that while cytoplasm alone may carry on neutral 

 or destructive processes, the performance of constructive processes re- 

 quires the presence of the nucleus. The evidence in support of this 

 statement comes from a variety of sources. 



Evidence From Regeneration. — One of the first and best of the indi- 

 cations of nuclear influence is the behavior of enucleated cell fragments. 

 One experimenter found that if Oxytricha (a protozoon similar to Parame- 

 cium) were cut in two in such a way that one fragment contained all the 

 nucleus, this nucleated portion quickly healed the wound and regenerated 

 the missing parts. The fragment without a nucleus, however, soon 

 perished. Stentor, another relative of Paramecium, showed similar 

 results. In this animal the nucleus is a long body like a chain of beads 

 (see Fig. 33), and it is possible to cut the body into several fragments 

 containing large or small portions of the nucleus, or none at all. Gruber 

 found that pieces containing much of the nucleus completely regenerated 

 in twenty-four hours; those with smaller nuclear fragments regenerated 

 more slowly; and those without any nuclear material, though the wound 

 closed, underwent no regeneration, and later died. Polj^stomella, an 

 Amoeba-like protozoon with a perforated shell, may likewise be cut into 

 fragments, one or more without nuclei. The piece with the nucleus is 

 able to repair the shell, while non-nucleated pieces lack this power. 

 Other experiments on Amoeba and upon other Protozoa lead to concor- 

 dant results. The non-nucleated portions may live and move for a 

 time, and their pulsating vacuoles may continue to pulsate with little 

 change; but they lack the power of digestion and secretion. 



