32 C. M. CHILD 



often 'totipotent/ as is evident from those cases in which the 

 segmental region remains capable, even in the adult, of giving 

 rise to a new head, as well as to all other parts, when physio- 

 logically or physically isolated from the head-region. 



The formation of the posterior growing region and of the segments 



From the work of Hyman ('16) on the oHgochetes and the 

 present paper on the polychetes, it appears that the metabolic 

 activity of the entoderm of that portion of 'the embryo or larva 

 which becomes the growing region increases more rapidly than 

 that of other regions during early development, so that, finally, 

 it becomes the most active region and is embryonic in character 

 and capable of long continued or indefinite growth. In the first 

 place the more rapid and more extreme rejuvenescence of this 

 region is doubtless associated with its less extreme specialization. 



In forms such as the oligochetes and particularly the leeches, 

 where teloblasts persist in the growing region, the physiologically 

 youngest region with the highest metabolic rate in both ecto- 

 derm and mesoderm is probably somewhat anterior to the teleo- 

 blasts, where the cells have begun to divide and grow. In any 

 case there is in all annelids at some stage a posterior, more or 

 less embryonic growing region of high metabolic activity. 



From this region segments arise successively, and the problem 

 of the physiological conditions and processes concerned in their 

 origin must be briefly considered. The facts suggest two possi- 

 bilities: one that the segment is essentially a new individual or 

 zooid, and the formation of segments a process of reproduction 

 which is inhibited by the physiological integration of the seg- 

 ments into a composite individual. The other possibility is 

 that segmentation is primarily a mesodermal process a repro- 

 duction or reduplication of coelom sacs, other duplications being 

 secondary to these. The morphological theories of segmentation 

 fall into two groups corresponding more or less closely to these 

 two conceptions. 



If the segment is a new zooid, the process of segment-formation 

 is undoubtedly the result of physiological isolation in the growing 

 region. As this region becomes more active, it becomes more 



