PRIMORDIUM DEVELOPMENT 



175 



Fig. 44. Stomatogenesis in relation to the posterior pole. 



A. Primordium site reversed in situ and mouthparts excised. 

 Regeneration anlage forms best mouthparts at its posterior end 

 (above) but oral formations are also induced at the other end by 

 adjacent posterior pole. Polarity of primordium prevails as 

 metachronal beat of membranelles proceeds from posterior to 



anterior (arrows). (After Tartar, 1956b.) 



B. Wide stripe patch implanted transversely in mid-region 

 develops a primordium but forms no mouthparts as does the 

 other anlage in the normal primordium site extending to the 



posterior end. (After Tartar, 1956b.) 



C. Lateral graft of an extra tail-pole induces abortive mouth- 



parts formation in the middle of the primordium. 



(1959) who emphasized that the symmetry of the induced mouth is 

 always that of the inducing pole, i.e., the anterior end of the primor- 

 dium shows a double curvature which results in its coiling in the 

 normal direction. This was also shown in fusion masses in which 

 for some reason mouthparts formed at the "wrong" end of the 

 primordium yet coiled in the normal direction (Tartar, 1954, 

 Fig. 11). The pattern of the posterior ectoplasm therefore deter- 

 mines not only that coiling and invagination shall occur but also 

 the direction taken. Uhlig also noted that in these double-ended 

 formations the original polarity is functionally dominant, for the 

 metachronal beating of the membranelles originates at the normal 

 mouth and progresses without interruption to the other. 



Several other observations gave evidence that mouth formation 

 depends on the geometric relationship of the anlage to the topo- 

 graphy of the cell. Ectopic primordia, developing in primordium 

 sectors grafted transversely across the lateral striping of the host 

 did not form mouthparts (Fig. 44B). Sometimes when primordium 

 sites or primordia were reversed in place there was not a bipolar 



