ZSS FIELDS AND GRADIENTS 



anterior cut surface, is then a function of the size and degree of 

 differentiation of the head, which in turn appears to be a function 

 of the activity of the regenerating tissue from which it was formed.^ 

 A similar state of affairs is found in the reconstitution of pieces of 

 the stem of Tubularia. Here the apical tentacles constitute the 

 dominant region. The size of the rudiments of these determines 

 the distance between these and the rudiments of the basal tentacles, 

 and can be modified experimentally. In one series of experiments on 

 stem fragments of a definite length, the average length of the pri- 

 mordia of the two sets of tentacles was reduced by 12 per cent, by 

 immersion in M/i 50,000 KCN, and 23 per cent, by immersion in 

 M/50,000 solution^ (fig. 136). 



In reconstruction from dissociated cells in Corymorpha, the 

 frequency of complete hydranths was much reduced and that of 

 partial forms, consisting of apical portions only, much increased by 

 moving the undifferentiated cell-aggregates about during a certain 

 critical time after their formation instead of leaving them attached 

 to the substratum. The interpretation advanced is that when 

 attached, the differential established between well-oxygenated 

 upper surface and poorly oxygenated lower surface will be large, 

 the resultant gradient steep; when moved, the gradient between 

 apical and basal regions will be less steep, and the structure can 

 therefore differentiate on a larger scale, whereas with a steep 

 gradient it is more compressed.^ In other words, the morphogenetic 

 field of the polyp in process of reconstitution can be altered 

 as a whole by altering the differentiation of the apical region.* 



Interesting results have also been obtained in Sahella (p. 165). In 

 abdominal fragments of this worm, the number of segments of 

 abdominal type which are transformed into segments of thoracic 

 type by a regenerated head varies from o to 75 (the number pro- 

 duced in normal ontogeny is 5 to 1 1). Here, the agencies responsible 

 for the wide range in the extent of the region morphogenetically 

 affected by the new head in this case appear to reside chiefly in the 

 old tissues^ (fig. 137). 



The conversion of abdominal into thoracic segments, obtained 

 experimentally in Sahella, occurs as a normal process in the develop- 



1 Child, 1915A. - Child, 1931. ^ Child, 1928 b. 



^ Child, 1915 A, p. 128. ^ Berrill, 1931. 



