Minimal size reduction in planarians through successive regenerations

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 Planarja 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 proportion 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. I n order to eliminate somewhat these factors the device was resorted 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 wTere 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 complete 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 restoration of the missing parts. As a general rule it may be said that the smaller the piece the more slowly the restorative processes take place. I n 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