Fig. 4. Examples of sector classes. Class I sectors have lateral boundaries either among M cells (A) or include a complete ring of BS cells (B). Class II sectors have boundaries that fractionate the BS cell ring (C). BS cells in an aberrant cell cluster in tan1 were without exception clonally related to at least one of the normal BS cells surrounding the adjacent vein (D, E). Neither D nor E depicts a section through a transverse vein, as determined by examination of serial sections.