A Method for Continuous Chromosome Control of Growing Rabbits

Abstract

IN experiments for inducing heteroploidy in the Mammalia, it is of utmost interest to follow the chromosome picture of a treated animal through the developmental period from immediately after fertilization of the egg until adult age has been reached, or at least until sexual maturity. Beatty and Fischberg¹ have described a method for obtaining mouse blastocysts for chromosome counts three and a half days after treatment of tubal eggs with cold- or hot-shock, that is, at the pre-implantation stage when the dividing cells usually contain large chromosomes. In rabbits it is most convenient to work with blastocysts at the age of 5–5½ days after copulation. At this age the diameter of the eggs is about 1–1.5 mm., but large variations are found. In older blastocysts, for example, six days old, it is often difficult to get good squash preparations due to the explosive breakup of the blastocysts, after which the different layers are difficult to spread out; although this can be done after some experience with a pair of needles. In two- to three-day old eggs, the individual cells are a little larger than from the fourth day, but the number of divisions is smaller. (This is a subjective view; no direct measurements of the cell diameter have been made.) The disadvantage in using blastocysts in studies of the chromosome number lies in the fact that it is very difficult to separate the trophoblast cells from the inner layer, and, so far as we know, nothing is known about the difference or variation in chromosome number from these different tissues. This work requires further examination.