The formation, growth, and conjugation of the pronuclei in the rat egg.

SYNOPSIS. A method is described whereby living rat eggs can often be observed under the phase-contrast microscope for as long as 12–16 hours. In this way it is possible to study in vitro the changes which occur during fertilization. The observations which have been made are described and illustrated, and time-relations are given for the changes in vitro. Information is presented on the manner in which: (a) the sperm nucleus changes to form the male pronucleus; (b) the second maturation spindle, initially in metaphase, progresses to form the female pronucleus, and is associated with the abstriction of the polar body; (c) pro-nuclear growth appears to pass through three stages characterized by the enlargement and coalescence of primary nucleoli, the enlargement of the pronucleus, and the appearance and growth of secondary nucleoli; (d) the pronuclei, when fully developed, come into contact and then diminish and disappear, giving place to the prophase chromosomes; (e) the chromosomes are arranged on the first segmentation spindle, which then undergoes division and is succeeded by the appearance of objects resembling early zygote nuclei. Some aspects of these observations are discussed in relation to previous descriptions of fertilization based on fixed and stained material, and also in relation to Caspersson's theory of protein synthesis.