320 CAUSES OF VARIATION 



6. If the cut be made beyond, that is posterior to, the middle, 

 both parts will regenerate. The posterior part, however, will re- 

 generate slowly and with difficulty, not head, but tail segments, 

 and the anomaly arises of an earthworm with two tails and 

 no head. There is thus a kind of "polarity" by which anterior 

 segments may arise only from comparatively anterior regions, 

 and by which posterior regions must of necessity give rise to 

 posterior segments. The same significant facts are notable in 

 other species, both animal and plant (see Fig. 36, /). 



7. " This same relation between the number of segments cut 

 off from the anterior end and the number that is regenerated 

 seems to hold good throughout the whole group of annelids, 

 although the maximum number that comes back may be differ- 

 ent in different species. Thus in Lumbriculus six, seven, or 

 even eight new segments come back, if more than that number 

 are removed." 1 There is therefore a definite termination to 

 the power of regeneration, even in species with high regener- 

 ating powers. 



8. " If we examine the method of regeneration from the pos- 

 terior end of a piece of an earthworm, we find that when several 

 or many posterior segments have been removed a new part comes 

 back, composed at first of very few segments," 2 the last of which 

 contains the new opening for the digestive tract. Later addi- 

 tional segments are formed just ahead of the last segment, quite 

 after the normal manner of increasing the length in many species 

 of annelids, 



9. The anterior portion does not possess the power of regen- 

 eration unless it consists of a considerable number of segments 

 (see Fig. 37, A-E). 



10. If a short piece (three to seven segments) of the anterior 

 portion of one worm be grafted in a reversed position upon the 

 anterior end of another worm, trie grafted part will regenerate a 

 head of about two segments from its free end, which was origi- 

 nally the posterior extremity, but of an anterior piece (see Fig. 

 37, F). This shows the entire power of reversal as to relative 

 position of segments, a high degree of adaptability. 



1 Morgan, Regeneration, p. 9. 



2 Ibid. p. 9. 



