EFFECTS OF COLCHICINE ON THE CLEAVAGE OF THE FROG'S EGG (RANA PIPIENS)

¹ From the Biological Laboratories, Radcliffe College, and Harvard University

The treatment of the developing frog's egg with colchicine (concentrations from 1: 10,000 to 1: 100,000) in amounts sufficient to retard mitosis or cause a temporary arrest of the process, results in a varying proportion of the eggs exhibiting a meroblastic type of cleavage. The degree of restriction of cleavage to the animal-pole region varies from the formation of a small cap of cells to a condition in which cleavage may include almost the entire egg. The level of development and completeness of the embryo are directly correlated with the extent of restriction of cleavage. Such animal-hemisphere embryos may not be capable of attaining even a modified blastula stage. In many blastulae only the roof of the segmentation cavity is cellular. The process of gastrulation is always greatly disturbed and in extreme cases may be limited to a modified involution. Frequently, neural plate with beginning neural folds, notochord, dorsal mesoderm and entoderm are differentiated. Embryos rarely develop beyond this level.

In many embryos there are scattered areas in which the cells are large and possess correspondingly large nuclei. These are tentatively interpreted as the result of tetraploidy, a phenomenon commonly produced in plants by colchicine. The free yolk-nuclei are characteristically abnormal.