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In their interesting paper Tilney and Byers (6) describe the arrangement of the microtubules constituting the axonemes of Echinosphaerium nucleofilum . The writer finds several points of theirs with which on geometrical grounds he cannot agree . Fig. 1 shows an idealized cross-section of an axoneme with the positions of the microtubules shown by heavy dots. The microtubules are arranged in a double polygonal spiral with the array divided into 12 sectors by 12 clearly defined, approximately radial boundaries. The boundaries radiate from two central microtubules . There is evidence for the presence of macromolecular links connecting microtubules but, unfortunately, in the only published micrograph known to the writer which distinctly shows links (6), the arrangement is far from clear.

THE ARRANGEMENT OF THE AXONEMAL MICROTUBULES AND LINKS OF ECHINOSPHAERIUM NUCLEOFILUM