PRIMORDIUM DEVELOPMENT I73 



Now it is very suggestive that it is just at this time that the 

 primordium becomes susceptible to sloughing when stentors are 

 treated with salt and other solutions (see p. 253). This shedding 

 response indicates that the anlage has become disconnectable from 

 the surrounding ectoplasm and this in turn suggests not only that 

 the primordium has become an integrated unit but also that it may 

 now be isolated from morphogenetic influences emanating from 

 its immediate surroundings. As will be developed shortly, there 

 are strong indications that mouthparts are induced at the posterior 

 end of the anlage by reason of its relationship to the surrounding 

 parts at the posterior end of the cell. 



Parenthetically it should be noted that in one salt treatment of 

 a stentor with a stage-4 primordium it happened that only a 

 section of the membranellar band in the region of the presumptive 

 oral pouch was sloughed, and again there was formed a perfect set 

 of feeding organelles except that the pouch was missing. Since the 

 ectoplasmic striping alongside the anlage is in these cases never 

 affected, it may be assumed that the lining of the presumptive 

 pouch had not been removed. From this it may be inferred that a 

 certain section or bend of the developing primordium induces 

 pouch formation in the adjacent ectoplasm on its right and that 

 in the absence of this influence this material remains unaltered. 



Progressive determination of the oral primordium is also demon- 

 strated by shifting its location on the cell. When stage-2 primordia 

 were circumscribed and shifted to the anterior pole they developed 

 without forming mouthparts (Fig. 43A), but when this operation 

 is performed upon later anlagen, oral development is then complete. 

 Early primordia shifted to the posterior end developed adequate 

 mouthparts and a well-formed though somewhat shortened mem- 

 branellar band (unpublished). In one case a grafted anlage 

 developing at the posterior pole even showed an additional gullet 

 formation (b). 



It therefore appears that the normal surroundings of the 



pouch only, with adequate gullet attached to a long peristome. 



F. Removal of large portions, whether anterior or posterior, of 



stage-2 primordium has no eff"ect ; remaining section of the anlage 



elongates and forms complete set of feeding organelles. (After 



Tartar, i957c.) 



