Pretreatment with isotonic solutions (both ionic and non-ionic in the case of the mouse, ionic only in the frog) generally produced a similar effect, viz., chromosomal swelling with little effect on the spindle. A notable exception is provided by frog embryos preceding the neurula stage; spindle disorganization without chromosomal swelling was produced by pretreatment in isotonic modified Niu-Twitty solution, containing no divalent cations.

Pretreatment with hypotonic solutions (both ionic and non-ionic in the case of the mouse, ionic only in the frog) generally produced several major effects, viz., despiralization of chromosomes, chromatid separation, and spindle disorganization. The conclusion is drawn from the mouse data that, in order to produce these effects, pretreating solutions must be of low osmotic pressure. Low ionic strength alone (e.g., isotonic sucrose solutions) is not sufficient.

As differentiation of frog embryos progressed, pretreatments either of longer duration or with solutions of increasing degrees of hypotonicity were required to produce comparable intensities of the same effects.

Many of the effects on metaphases produced by hypotonic pretreatment of frog embryos were reversible by subsequent exposure to isotonic solutions.

The significance of results presented here is discussed briefly with respect to some current considerations of the macromolecular structure of chromosomes.