176 THE BIOLOGY OF STENTOR 



differentiation, as described above, but instead no mouthparts 

 were formed at all. In these cases, as in the absence or incomplete- 

 ness of oral formation in stentors grafted in complex, random 

 orientation, the failure of oral differentiation may be attributed to 

 the mutual cancellation of polar gradients (Tartar, 1956b). 



Starting from these impHcations, Schwartz's student, Uhlig 

 (1959, and unpublished thesis), has pursued this matter in a 

 demonstration of morphogenetic gradients in Stentor coeruleus, 

 suggestively similar to those which have been postulated for the 

 cleaving sea urchin egg. To mention only two of his experiments, 

 Uhlig found consistently that when primordia were grafted trans- 

 versely across the axis of the cell they never formed mouthparts, 

 suggesting that although the anlage arises in this manner the 

 assumption of its later anterio-posterior position is for more than 

 *' historical " reasons, namely, to align it with a morphogenetic 

 gradient which will insure its complete and proper development. 

 He also found that when an extra tail pole was grafted laterally 

 alongside a developing primordium this had the effect of inducing 

 an additional if somewhat incomplete formation in the middle of 

 the anlage (Fig. 44c). The interpretation that this induction may 

 be due to the operation of some gradient steepest at the posterior 

 pole will be discussed in a later section on polarity (p. 202). 



Hence there is good evidence that the oral anlage is induced to 

 form mouthparts by its normal surroundings. Once this inter- 

 action has taken place and although there are yet no beginnings 

 of mouthparts, the primordium is then determined and can 

 develop completely regardless of where it is placed. 



5. Repair, mending, and joining of primordia 



The oral anlage can endure drastic cutting injuries without 

 total blockage of development (Tartar, 1957c). For example, a 

 stage-3 primordium was transected in many places yet produced 

 an apparently complete set of feeding organelles, though these 

 were later replaced by a new set (Fig. 45A). Usually, however, this 

 operation, or the comparable one in which the whole length of the 

 anlage is slashed through several times with the point of a glass 

 needle, generally did not prevent the formation of a good mem- 

 branellar band but the mouthparts were lacking (b), especially 

 when later-stage primordia were used. As in continued develop- 



