196 BIOLOGICAL LECTURES. 



I. 



Numerous experiments on Protozoa and Protophyta have 

 shown that a nucleated part is capable of forming a new indi- 

 vidual. So far as we can see there is not in most cases, perhaps 

 in none, the formation of new indifferent protoplasm in which 

 the new parts are developed, but the entire piece is changed 

 over into a complete animal or plant of smaller size. At first 

 sight there seems to be here a marked difference between the 

 regeneration of unicellular and multicellular forms, for in the 

 latter it is usual for a knob of new tissue to appear, and out 

 of this the new part develops. However, Trembley saw that 

 when a Hydra is cut longitudinally the cut edges bend in and 

 fuse, forming a new tube of smaller diameter. Nussbaum has 

 also observed in Hydra the rolling in and fusion of the cut 

 edges. In both cases the new form develops without the 

 previous formation of new tissue. In a tubularian hydroid 

 Bickford has found that when a piece is cut from the stem the 

 new tentacles appear in the old tissue, and I shall describe 

 more fully below the results of some experiments on planarians 

 which show that the old part plays an important role in the 

 formation of a new individual. We see then that the difference 

 between unicellular and multicellular forms is not so great as 

 appears at first sight. The special problem that we meet with 

 here is this : How far are the processes of regeneration in uni- 

 cellular and multicellular forms the same ? Can we compare 

 with the Protozoa only those multicellular forms in which 

 regeneration takes place directly in the old parts ? If so, is the 

 relation between the cells only one of correlation, as Hertwig 

 has recently urged for all multicellular forms ; or are we still 

 prejudiced by the cell-dogma ? Do not these cases of regenera- 

 tion in the multicellular form indicate that the individual is a 

 whole in the same sense that the unicellular form is a whole ? 



II. 



In the regeneration of the lower animals and lower plants we 

 do not find any differences that need detain us, but a curious 



