296 FIELDS AND GRADIENTS 



the detachment of the part of the chain dominated by this second 

 head only takes place when a certain greater distance has been 

 reached. The formation of new zooids in colonial organisms such 

 as hydroids is regulated in relation to this rule. The distance be- 

 tween zooids — i.e. the extent of the field controlled by a more apical 

 zooid — varies with temperature, nutrition and other conditions.^ 



Complete physiological isolation of an incipient new apical re- 

 gion can also be achieved by removing the old dominating apical 

 region. Some species of Planaria reproduce by transverse fission. 

 By cutting off the original head, precocious fission is induced. 

 Further, in these forms, the length of body attained before fission 

 occurs varies with the degree of differentiation of the head : if as a 

 result of regeneration in depressant solutions a subnormal head is 

 produced, it can only control a small field, and fission occurs at an 

 unusually small body-length. ^ 



In the regeneration of such forms as hydroids and worms, 

 various complications may be found. Sometimes biaxial regenera- 

 tion occurs, leading to the formation of two apical regions, one at 

 each end of the piece (or, more rarely, two basal regions — e.g. tails 

 in Planarians). Sometimes no apical organ is regenerated. The 

 percentage frequency with which this occurs almost always in- 

 creases with increasing distance of the front end of the fragment 

 from the original front end of the body. When an apical region is 

 regenerated, its final form and the rate of its regeneration also vary 

 with the level of the original body from which regeneration takes 

 place.^ 



It is unnecessary to go into detail here as to the reasons for these 

 complications. They appear to depend on the interplay of several 

 factors. The result depends in the first place on the portion of the 

 gradient-field of the original body contained in the cut fragment. 

 Secondly, on the release of the fragment from the inhibition ex- 

 erted by the old dominant region, which results in an increase of 



1 Child, 1 929 A. 



2 In plants, physiological isolation has been obtained by exposing to low 

 temperature a portion of the region (e.g. a runner) connecting dominant and 

 subordinate parts. Even though under these conditions the runner continues to 

 grow, the field is interrupted, and a new plant is precociously formed at the free 

 tip of the runner. Similar experiments have not yet been successfully carried 

 out on animals. See Child, 1915 A. 



2 Sivickis, 1931 A and b. 



