172 



THE BIOLOGY OF STENTOR 



Fig. 42. Developmental determination of the oral primordium. 



Removal of portions of the stage-4 anlage results in corres- 

 ponding deletions : A. Posterior half produces mouthparts and 

 short membranellar band. B. Anterior two-thirds forms no 

 mouthparts, only peristome. C. Posterior fourth forms only 

 gullet with opening. 



Removal of parts at stage 5, before stomatogenesis has visibly 

 begun. D. Excision of posterior tip of anlage results in absence 

 of gullet and cytostome, only membranellar band and oral pouch 

 being formed. E. Penultimate deletion results in absence of oral 



