PROCESS OF REGENERATION 155 



amount of independence. One may say, then, that there is an 

 antagonism between the primary and the secondary gradient, 

 the one making for subordination of the posterior region, the 

 other for independence. The primary gradient, however, al- 

 ways has the upper hand, for it is there from the very beginning, 

 and each new segment which arises is forced, despite its high 

 metabolic rate, to become part of the system of conduction, 

 which has already been long established in the antero-posterior 

 direction. The secondary gradient probably aids in the inhibi- 

 tion of head formation, but is unable otherwise to influence the 

 morphogenetic effect of the primary gradient, unless the latter 

 is eliminated by other factors. If this were not so, it would 

 be impossible to understand why heads do not arise at the poste- 

 rior end of long pieces of Lumbriculus and Tubifex. I might 

 further point out that the zooids of the naids arise without regard 

 to the secondary gradient, and often within it, and that the 'break- 

 ing' region of Lumbriculus is always within the secondary rise; 

 both of which facts indicate that the secondary gradient offers 

 very ineffective opposition to the primary gradient. For zooid 

 formation is, as Child has shown ('11 a), a matter of physiological 

 isolation from the primary gradient, usually as a result of de- 

 creased intensity of correlative stimulation, and would be 

 inhibited by the presence of a gradient running in the other 

 direction. 



In how far do these conceptions, developed from the data on 

 Lumbriculus, explain the facts of regeneration in the other forms? 

 Why does Dero form normal heads at all levels? Experiments 

 on the rate of metabolism of pieces of Dero at various times after 

 section have shown that the rate is very high immediately after 

 section, but begins to fall at once. Within three hours it has 

 fallen below that of the corresponding part of the whole worm, 

 in five hours it is still lower, and continues to fall until about 

 twenty hours, after which a permanent and increasing rise in 

 rat%sets in. While I have not been able to devise any method 

 for determining the time of head determination in Dero, yet it 

 certainly is most improbable that it could occur during the very 

 brief period of stimulation. One may therefore safely say that 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 20, NO. 2 



