Studies on Regulation. 501 



concerned in this difference: in the first place Series 71 was begun 

 October 29 and Series 79 January 9. At the first date the tem- 

 perature of the water was much higher than on the second and all 

 specimens were much more active. The regenerative power in 

 pieces with ganglia differs markedly with season, /. <?., with tem- 

 perature, as is shown by comparison of Figs. 4 and 5 and 40 

 and 46, both of which are summer experiments, with Fig. 11, a 

 winter experiment, from nearly the same level. Pieces without 

 ganglia may be expected to show similar though less marked 

 differences. 



It is possible that another factor is also concerned: in Series 71 

 the anterior cut was made as near the ganglia as possible (Fig. 28, 

 dotted line), while in Series 79 it was somewhat further posterior 

 (Fig. 12), the chief object being in this case to remove all parts 

 near the ganglia. In the consideration of anterior regeneration 

 it will appear that the region immediately posterior to the ganglia 

 differs to some extent from other portions of the cords, since its 

 presence determines greater activity and more anterior regenera- 

 tion; posterior regeneration is probably similarly affected by the 

 presence or absence of this region. It may be, therefore, that the 

 difference between Series 79 and 71 is due in some degree to the 

 absence of this region in the one case and its presence in the other. 



d. Posterior Regeneration in the Head Region. 



In all cases of posterior regeneration thus far described the level 

 from which regeneration occurred was one millimeter or more 

 posterior to the ganglia. The piece whose history is given in 

 Figs. 1-5 was cut about a millimeter posterior to the ganglia. 



It was found, however, that results differed very little whether 

 the cut surface was one millimeter posterior to the ganglia, or 

 immediately behind them. So long as the ganglia were not greatly 

 injured by the cut, regeneration took place essentially as in Figs. 

 1-5. But when the cut removed or injured considerable portions 

 of the ganglia the amount of regeneration was only slight and in 

 many cases there was no visible differentiation in the regenerated 

 tissue. The history of one series is given in some detail as an 

 example of these experiments. 



