CHAPTER 9 - Informative Variations on the Normal Process

Summary

Unequal Division

The usually orderly and predictable pattern of division that occurs during plant and animal development led to generalizations like Sachs's rules and Hertwig's modification of Sachs's rules (discussed in Wilson, 1928). These generalizations reflected the idea that the pattern of divisions is affected by the size and proportions of the protoplasmic mass that is undergoing division. Wilson (1928, p. 982) translated Sachs's rules as follows: “(1) Cells typically tend to divide into equal parts. (2) Each new plane of division tends to intersect the preceding one at right angles.” O. Hertwig's modifications related the statements more closely to the circumstances in animal egg cleavage patterns (Wilson, 1928, p. 984). “(1) The typical position of the nucleus (and hence of the mitotic figure) tends toward the center of its sphere of influence, i. e., of the protoplasmic mass in which it lies. (2) The axis of the spindle typically lies in the longest axis of the protoplasmic mass, and division therefore tends to cut this axis transversely.” The general subject interested embryologists at the turn of the century because they had many opportunities to observe the relationships under ideal conditions, and because the cleavage pattern appeared to play an important role in the normal distribution of “formative substances” in the early development of some major animal groups.