164 THE BIOLOGY OF STENTOR 



(Schwartz, 1935). As the two ends of the new membranellar band 

 approach each other the enclosed fine stripes are bent into arcs in 

 the frontal field. 



There are certain modifications of the primordium in different 

 types of morphogenesis. Speaking teleologically, the primordium 

 in reorganization needs only to replace the original mouthparts 

 which will be dissolved, and, as Schwartz emphasized, there is 

 accordingly less multiplication of fine stripes than in either 

 division or regeneration. In dividers the anlage, already at stage-2, 

 may appear semicircular as its anterior end also bends to the right 

 and cuts across lateral striping, and stripe multiplication may be 

 observed along both ends (see Fig. 15A). On the contrary, in 

 regenerators and reorganizers the anterior end of the primordium 

 usually runs straight forward to the anterior pole or to a pre- 

 existing adoral membranellar band. The D-shaped primordium 

 is usually diagnostic of dividing animals. All primordia begin in 

 stage I at the same site and level of the cell. This was remarked in 

 reference to regenerators and reorganizers by Johnson. My im- 

 pression is that it is also true of dividers, Weisz (1951b) to the 

 contrary. 



Earlier students of Stentor in the heyday of the recapitulation 

 theory saw an evolutionary significance in the lateral origin of the 

 oral primordium in stentors. Both Schuberg (1890) and Johnson 

 (1893) regarded this as a return to a more primitive design in 

 heterotrichous ciliates. Spirostomum, presumably more primitive, 

 retains the lateral disposition of the membranellar band and 

 resembles regenerating stentors in stage 6. In Fabrea the mouth- 

 parts would be shifted half-way forward from their original 

 posterior location having the appearance of stage 7 in stentors. 

 In Stentor itself they would eventually achieve the wholly anterior 

 disposition. Finally, in Folliculinids the highest development 

 would be achieved, in which there is an enormous extension of 

 the membranellar band in two folds projecting outward from the 

 anterior end. 



2. Primordium development under abnormal conditions 



Such is the normal development of the oral primordium so far 

 as we now know. Its behavior under unusual circumstances may 

 give us some insight into the processes involved (Tartar, 1958b). 



