Mechanism of Anomaly Induction in Frog Eggs by Means of the Centrifuge

Abstract

The mechanism of the production of anomalies by the centrifuge has taken on new significance since the work of Spemann and his school on the organizer and related phenomena. Apparently few investigators have studied centrifuged eggs in the light of this work. The eggs of Hyla regilla, the tree frog, have proven especially sensitive to centrifugal force, yielding highly specific accessory structures (tails and ectodermal proctodea) in as high as 100'% of the specimens, if treated at the correct stage of development. Electric and hand centrifuges were used. High percentages of accessory tails and ectod. ani appear after application of centrifugal forces corresponding to 1300-2000 revolutions per minute, continued for periods ranging from 5 to 120 seconds (centrifuge arm, 11.5 cm.). Susceptibility to these forces of the centrifuge is very limited. Secondary tails and ani are not observed until the eggs treated are advanced blastulce (about 3 hours before the onset of gastrulation). From this time on the percentage of anomalous embryos increases, reaching a maximum (90-100%) at about the time when the blasto-pore first becomes visible. Susceptibility then drops rather abruptly and is nil at about 2 hours after the onset of gastrulation (at temperatures 25°-28° C).

Secondary tails present a great variety of structures: some are quite complete with chorda, somites, nerve tube, mesenchyme, secondary anus (Fig. A); others contain only a solid axial strand (Fig. C); others consist of a flat fin with axial strand at one edge (Fig. B). Secondary ectod. ani are common (Figs. A, B).

Clear secondary tails and ectod. ani were induced by other means: (1) by pressing the presumptive ectoderm of susceptible eggs against the yolk mass, (2) by implantation of ventral or lateral blastoporal lips of the early gastrula into the blastocoele.