PRIMORDIUM DEVELOPMENT l6l 



that there is nothing magically restrictive about the ramifying 

 zone as far as primordium formation is concerned. 



In fact, Morgan (1901a) soon found that oral regeneration 

 occurred readily enough even after the normal site of the anlage 

 was removed, and Stevens (1903) confirmed this by showing that 

 in longitudinal aboral halves lacking this site entirely the oral 

 primordium appeared in the line of heal. Faure-Fremiet then 

 posed explicitly the question of whether, if the primordium always 

 appears at the same place in stentors, there is some specialized 

 potential restricted to this area; but his student Causin (1931) 

 likewise found that the normal primordium site could be com- 

 pletely eliminated without preventing regeneration. Therefore 

 there are not localized potentialities for oral differentiation in one 

 region of the cell. This point has been amply confirmed by later 

 students of Stentor, including myself. Weisz regarded the oral 

 primordium as arising from a single stomatogenic kinety next to 

 the left boundary stripe of the ramifying zone. He stated (1953, 

 1954) that not only tiny fragments but also pieces larger than half 

 the cell can be cut which do not regenerate because they lack the 

 specialized kinetosomes of this meridian ; although reporting that 

 longitudinal aboral halves can regenerate and that in division the 

 primordium bends so that it eventually touches the left boundary 

 stripe, from which it follows that the anlage originates away from 

 this stripe. That the ectoplasm is virtually totipotent throughout 

 as regards oral differentiation will become even clearer as our dis- 

 cussion proceeds, and this is not contradictory to the fact that the 

 oral primordium usually appears in a certain place. 



Stages of visible change in the anlage in regeneration have been 

 defined (Tartar, 1957c) and are altogether comparable to those 

 of division (Fig. 40). The first sign of primordium formation as 

 seen in coeruleus is a scooping of the pigment granules to each side 

 as a rift crosses about 10 granular stripes (Moxon, 1869). A groove 

 with slightly projecting flanges is evident at later stages in cross- 

 sectioned view, as shown. Stripe multiplication also occurs with 

 the splitting of granular bands both immediately above and below 

 the primordium. The primordium extends from both ends, 

 cutting across more stripes posteriorly, the anterior end reaching 

 forward. Now the anlage has a glistening appearance, presumably 

 due to cilia growing out from kinetosomes included within it. 



