IQ6 C. M. CHILD. 



logically isolated from the dominant region (Child, 'na, 'nd). 



That the occurrence of fragmentation is connected with a 

 decrease in the rate of metabolism and consequent physiological 

 isolatiion of posterior regions is clearly indicated by the fact that 

 fragmentation may often be induced, even in worms which are 

 not full-grown, by suddenly lowering the temperature ten to 

 fifteen degrees. In such cases fragmentation usually begins in 

 the posterior region within a few days. 



The degree of isolation is not sufficient to permit development 

 at once into a new individual but it is sufficient to permit some 

 degree of independence in motor reaction, consequently, at some 

 time when the worm is creeping the posterior end attaches itself 

 and the rest of the body pulls away from it, as in P. dorotocephala. 

 Apparently the greater part of the body in old fragmenting 

 animals consists of a series of these small zooids for in most 

 animals fragmentation continues until only the anterior third or 

 fourth of the body together with the head remains. This 

 anterior piece may then encyst or may undergo rejuvenescence 

 without encystment and after some weeks give rise to a new 

 posterior end, or in some cases it dies. 



The posterior zooids are present only dynamically and not mor- 

 phologically, at least not visibly, and they are not to be thought 

 of as absolutely fixed stable entities. When the animal is 

 strongly stimulated it is able to control the whole length of the 

 body and for the time being the posterior zooids may almost or 

 quite cease to exist, only to reappear after the stimulation is over. 

 When such zooids are established the regions at their ends must 

 be subjected to constantly varying correlative conditions. 

 Sometimes they may form a physiological posterior part of one 

 zooid, at other times an anterior part of another and at still 

 others a part of neither. Such changes in correlative conditions 

 must tend to weaken and eliminate the existing structure in 

 those regions since the development of such structure depends on 

 a certain degree of constancy in correlative factors. In this way 

 zones of structural weakness arise and these are the zones where 

 separation occurs. 



Occasionally, either in consequence of weakness or perhaps 

 because the physiological isolation of the posterior regions is 



