Pattern stability in the insect segment : II. The intersegmental region.

1. InDysdercus the abdominal segments are isolated from each other by an intersegmental region which can be distinguished on the basis of morphological and physiological criteria. 2. The intersegmental region (ISR) consists of the visible segment border and narrow strips of cells anterior and posterior to it. 3. The anterior strip (w-ISR) is white and merges with the white segment region (wS) in front of it, and the posterior strip (r-ISR) is red and merges posteriorly with the red segment region (rS). The wS and the rS meet in the middle of the segment; together they form the segment proper. 4. Grafts from the ISR have been transplanted to various positions within a segment. The reactions of graft and host, respectively, can be distinguished in combinations involving a colour mutant and/or individuals of different sexes. 5. The results show that cells of the r-ISR and the w-ISR each have some adhesiveness towards those tissues which they border in situ, and less adhesiveness towards other tissues. That is, the w-ISR is adhesive towards the r-ISR and wS and is usually rejected by tissue of the rS, whereas the r-ISR is adhesive towards tissue of the w-ISR and rS, but is rejected by tissue of the wS. 6. The role which the ISR plays as a barrier between adjacent segments can essentially be interpreted on the basis of differences in cell adhesiveness. 7. Besides these adhesiveness properties the two parts of the ISR show a long-range influence on polarity and pigment synthesis in surrounding segment tissue. 8. The adhesiveness properties of the r-ISR and w-ISR can explain why the segment boundary forms such a straight line and why the ISR tends to grow between tissues from non-contiguous segment levels. This property can explain the hitherto not understood healing capacity of the ISR which even after wounding prevents cellular interactions between adjacent segments so effectively.