MINIMAL SIZE REDUCTION IN PLANARIANS 
THROUGH SUCCESSIVE REGENERATIONS 
S. J. HOLMES 
It is a familiar fact that very small pieces of fresh water plan 
arians will regenerate and give rise to minute individuals closely 
resembling the original form. I have endeavored to ascertain 
how far reduction of size in Planaria maculata may be carried 
without causing a failure to give rise to a normal individual. 
With very small pieces of an adult planarian there is a large pro- 
portion of cut surface which produces an injurious effect and there 
are also various mechanical impediments to regeneration; these 
facts, combined with the specialized condition of much of the 
tissue, conspire to restrict the regenerative capacity of the parts. 
In order to eliminate somewhat these factors the device was re- 
sorted to of subjecting the animals to a number of successive 
divisions. A planarian was cut into fifteen or twenty pieces; 
when these had regenerated into small planarians they were 
again cut into several pieces. These regenerated into still smaller 
individuals which in turn were divided, the process being con- 
tinued until forms were reached which were so small that com- 
plete regenerations were no longer obtained. In this way, when 
the minimal size limit was approached, regeneration became very 
slow and many of the pieces lived for months without restora- 
tion of the missing parts. Asa general rule it may be said that 
the smaller the piece the more slowly the restorative processes 
take place. In this way the proportion of cut surface was reduced, 
the tissue kept in a more plastic condition, and the whole process 
of regeneration made easier. 
Eugene Schultz has studied the reduction in size of planarians 
from starvation. He found that planarians could be reduced in 
this way to one-tenth or one-twelfth of their original size. A 
study of the size of cells of various kinds showed that there was 
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