CHAPTER IX 



PRIMORDIUM DEVELOPMENT 



An acutely felt omission in our data on Stentor is the lack of 

 silver-stain or electron micrographic studies of the developing 

 oral primordium. We have therefore no idea of what happens on 

 the level of fine structure during the most dramatic act of cyto- 

 differentiation. Yet much can be said in simple description of the 

 forming anlage and its relation to the pattern of lateral striping. 

 This relationship is two-fold: first, some of the ectoplasmic 

 stripes and bands adjacent to the primordium join with it to 

 complete the integrated parts of the ingestive apparatus, and 

 second, the anlage arises in definite correlation with the topo- 

 graphy of the cell surface. 



I. Normal location and development of the primordium 



At its inception the oral primordium seems to violate the 

 cortical pattern because it makes its appearance as a break in the 

 ectoplasm, cutting across the striping. The unpigmented rift sug- 

 gested to Johnson that the primordium originates in the endoplasm 

 and breaks through to the surface. He further argued that the 

 ectoplasm is too thin to supply the materials needed for this 

 extensive elaboration, besides being too highly differentiated to 

 participate in such ** embryonic" formations. In the related 

 Bursaria triincatella, Schmahl (1926) also found that the primor- 

 dium gives the appearance of breaking through the ectoplasm, yet 

 his cross-sections clearly showed him that the anlage lay entirely 

 in the surface. On the basis of other ciliate studies (see Lwoff, 

 1950) it is probable that the anlage is formed entirely in the ecto- 

 plasm and requires cortical derivatives such as kinetosomes for 

 its composition. Villeneuve-Brachon (1940) described accumula- 

 tion of kinetosomes in the early primordium, and these, in Stentor 

 as in the related Fabrea, seem to arise by multipHcation of granules 

 in the existing ciliary rows. 



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