THE PHYSIOLOGY OF REGENERATION 329 



weight (ill the left pan) by the receptacle and the necessary- 

 additional weights. When the object to be w^eighed has been placed 

 in the receptacle, the balance is again brought to equilibrium by 

 removing weights from the right-hand pan. The weights re- 

 moved equal the weight of the object. 



In studying the water content in regeneration one encounters a 

 very serious difficulty owing to the impracticability of severing 

 the. regenerating tissue from the organism in the early stages ; in 

 later stages, too, when the operation is more or less feasible, the 

 loss of water by evaporation in the course of manipulating the 

 organism, and the impossibility of separating accurately the new 

 from the old tissue would be sources of considerable error in the 

 final determinations, so that the results could not be very reliable 

 unless the regenerating organ were of considerable size. All these 

 difficulties were fully realized in the case of Podarke, a worm 

 scarcely more than three quarters of an inch in length, where the 

 regenerated part could be detached and examined by itself 

 neither immediately after the operation nor even at later periods. 

 Under the circumstances it became necessary to study the entire 

 regenerating animals at various intervals after the operation. 

 Moreover, the animals being too small to be studied separately 

 with a satisfactory degree of accuracy, it was necessary to examine 

 rather large numbers of worms at one time, using the average value 

 for the group as an indicator of the water content at any par- 

 ticular stage. 



I am quite aware that serious objections may be raised against 

 this procedure, the strength of which no one, perhaps, could feel 

 more keenly than I myself do. For this reason I shall enumerate 

 the various objections to which the present investigation is 

 justly open, and shall then attempt to show that, notwithstanding 

 the objections, the results still warrant the conclusions. 



One of the important defects of any study of this nature is the 

 lack of a continuous series of results based upon one and the 

 same group of individuals. It is obvious, of course, that a par- 

 ticular group of worms could be used to determine the condition 

 of the water content at only one stage in regeneration,^ but in 



^ To be sure, other branches of biological research share this disadvantage, 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 10, NO. 3 



