Rate of Regeneration in Cassiopca xamachana. 89 



be mentioned that these calculations have been made, according to the 

 nsual custom, from linear measurements only, but it must be borne in mind 

 that the actual volume of new tissue might oftentimes be more in a short 

 stocky bud than in a longer slender one. Some of the regenerating arm- 

 buds are short and branching, while others are long and simple in structure. 



There is no doubt that the individuals with six and seven removed arms, 

 those of the greatest degree of injury, regenerated at a faster average rate 

 than the other medusae of the series. It should be remembered that these 

 were the smallest, and, as a matter of fact, in all of the experiments the 

 small young medusae showed more ability to regenerate rapidly than did 

 larger old ones. Unfortunately this point was not controlled, although in- 

 directly it is checked by a comparison with the other members of the 

 series which are practically of a common size. In the last column, for in- 

 stance, those medusae with three and four removed arms are regenerating 

 at a better rate than those having lost five arms, and they are also slightly 

 smaller than the latter, although they were all of practically the same size. 



The objection to this experiment, which has probably suggested itself be- 

 fore this time, is that too few individuals were employed. This objection 

 is not as serious as it may seem at first sight. If among animals any such 

 general law of regeneration exists as that the greater the degree of injury 

 the faster the rate of regeneration, it should at least manifest itself to an 

 evident degree. It is not necessary to use more than half as many medusae 

 to satisfy one's self that the rate of degeneration is fastest from the middle 

 of a straight-cut surface and slowest from its outer corners ; that regenera- 

 tion is faster from a level 20 mm. from the disk margin than from any 

 level less than that distance from the margin ; that regeneration takes place 

 fastest from the widest end of a peripheral bias-cut strip of the medusa 

 disk and slowest from the narrow end of the strip; or, that a number of 

 other conditions will follow, depending upon the manner in which the disk 

 is cut. These are all facts in regeneration, and a number of them are 

 shown by distantly related animals; and, if the degree of injury determines 

 the rate of regeneration to any significant extent, it seems to the writer 

 that it should manifest this determination in a more evident manner. 



A serious objection to the view of a connection between the degree of 

 injury and the rate of regeneration is the fact that from a single medusa 

 the several arm-buds grow out at rates differing as widely as the average 

 rates of individuals with different degrees of injury. It is difficult to under- 

 stand how such a fact can be reconciled to the degree of injury idea. When 

 one individual has six arms cut away, each at the same distance from its 

 base, then it would be expected that all of the arms should regenerate at 

 almost the same rate, for there is little doubt that the degree of injury is 

 the same in each case and the surrounding conditions are as near as pos- 

 sible identical, being similar places on the body of one individual. 



