KARYOKINESIS DURING CLEAVAGE OF THE ZEBRA FISH BRACHYDANIO RERIO

¹ From the Department of Biology, Brown University

The nuclei in the blastomeres of Brachydanio rerio can be observed easily in life. They are visible in the prophase and telophase as well as in the interkinetic phase. This discovery is used (1) to confirm and consolidate the results obtained from sectioned material; (2) to fix the blastomeres in any desired mitotic phase; and (3) to determine the duration of mitosis and its phases.

The duration of mitosis and its phases under constant conditions, particularly with respect to temperature, is found to be constant for each cleavage. The time from the breakdown of 32 nuclei to the breakdown of 64 nuclei is 18 minutes at 25° C. This places the cleavage divisions of the zebra fish among the most rapid ever observed. The first six cleavages show a trend towards acceleration, the sixth being the most rapid one. From then on the speed of the divisions slows down. This trend is essentially undisturbed by changes in temperature.

The nuclear divisions during cleavage are characterized (1) by the formation of chromosomal vesicles in the telophase (some of these vesicles can frequently be seen in life to persist through the interphase); (2) by a comparatively short duration of this type of telophase; (3) by a very short, if any, true interphase; (4) by the lack of nucleoli; and (5) by the absence of a typical spireme formation in the prophase.

The very short duration of the telophase has been recorded for the first time. The other observations have been found in the cleavage divisions of a majority of the species examined. In the zebra fish egg they continue until about the twelfth cleavage, when the form of mitosis typical for the adult first appears. It is suggested that this type of mitosis is probably associated with the rapid sequence of divisions and is generally characteristic of cleavage mitoses. The most characteristic feature of this type of karyokinesis is the formation of the chromosomal vesicles, but the shortening of the interphase and telophase, and the lack of spireme formation in the prophase are also obvious.