NUCLEAR CHANGES IN RANA CLAMITANS. 121 



If (2) (migration of cells) were the method, we might find no 

 dividing cells at all, but should expect to find that the cells from 

 B to A or possibly only from D to A are turned with their long 

 axes parallel to the longitudinal axis of the spinal cord as if moving 

 toward the cut end. If (3) (division of more anterior cells in situ) 

 were the method, we should expect to find dividing cells all the way 

 from B to C or possibly concentrated in a growing zone ED. 



The present paper aims to give an account of the nuclear 

 changes, both degenerative and regenerative, involved in the 

 formation of the regenerated spinal cord. 



II. Material and Methods. 



Serial sections were made of tadpole tails killed after various 

 regeneration periods. This enables one to follow the process 

 from stage to stage. But to get uniform results from this method 

 and eliminate individual variations, one must take tadpoles as 

 nearly alike as possible at the start, operate on all at the same 

 time, keep them under uniform laboratory conditions and make 

 sections of several individuals at each stage. 



On October 12, 191 3, seventy tadpoles of Rana clamitans, 

 varying in length from 30 to 60 mm., were brought into the lab- 

 oratory. Two days later they were put into individual finger 

 bowls, and forty-four medium sized individuals (32-40 mm. in 

 length), chosen to constitute the main series, were grouped by 

 twos or threes. Those of each group were as nearly alike as 

 possible and each group was treated as a unit in the time of 

 operation, killing, etc. The finger bowls were placed side by 

 side on a table some distance from the windows so that uniform 

 conditions of temperature, light, etc., were insured. None of 

 the tadpoles was fed during the course of the experiment, and 

 none died from the effects of laboratory conditions. 



On October 15, the first operations were performed. Each 

 tadpole was transferred from the finger bowl to a paraffin block 

 and approximately one fourth of the tail was removed, with 

 a sharp scalpel, at right angles to the plane of the tail. The 

 animal was returned to the finger bowl and the removed part put 

 into Gilson's killing fluid. At the end of the period of regener- 

 ation, the animals were again taken out onto the block and the 



