PRIMORDIUM DEVELOPMENT 169 



may occur both above and below it with a single mouth produced 

 only in the lower segment (Fig. 41c). In one unusual instance, in 

 which the cell was cut in two longitudinally and the halves rotated 

 180° on each other, an adventitious primordium appeared first as a 

 series of islands of oral cilia which later connected to produce a 

 good membranellar band though mouthparts were not formed (d). 

 These cases suggest that primordium formation is the result of 

 local episodes of elaboration along its length rather than a 

 differentiation proceeding from a single center. 



Other observations indicate that dissolving or etching of formed 

 parts may be involved in the later stages of primordium develop- 

 ment, as if in this way space is provided for the evolving parts. 

 Thus in Bursaria truncatella (Schmahl, 1926) and in Condylostoma 

 magnum (Tartar, 1941b) the oral groove seems to be scooped out of 

 the cytoplasm by an active process of dissolution. Something like 

 this was seen in the development of an unusual stentor primordium 

 in the form of a pinched ring. The enclosed striping was dissolved 

 and cut out as a pendent tongue of cytoplasm (e). In two cases the 

 breakdown or dissolving of ectoplasmic structures along the whole 

 length of the primordium was conspicuous, and one of these is 

 shown (f). This was in a tandem graft of two Stentor coeruleus in 

 which, at stage 6 in development of the joined primordium, a wide 

 break in the striping was seen to the right so that one could clearly 

 see through the transparent plasma membrane into the cell interior, 

 and the gap was later covered by scattered pigment granules not in 

 rows. This may have been an exaggerated picture of what happens 

 when a place is provided for the clear border stripe to the right of 

 the membranellar band, as well as a demonstration that pigment 

 granules tend to invade any open or unstructured area of ectoplasm. 

 The case is also reminiscent of normal events in primordium 

 formation in Folliculina^ already mentioned, in which a gape or 

 spreading of the longitudinal striping to a distance of 15/x occurs. 

 When primordium formation occurs without stripe multiplication 

 there is sometimes the appearance at stage 5 of a dissolving of the 

 ectoplasm near the posterior end of the anlage (g) and this might 

 be regarded as a localized etching of the structured cortex to permit 

 the inward invagination of the anlage in gullet formation. 



Lengthening of the primordium can be blocked by incompatible 

 striping. Thus when cell and primordium were cut transversely 



