Results Zebrafish embryos treated with retinoic acid at concentrations of 10-11 M and 10-10 M exhibited no apparent developmental abnormalities. By approximately 24-hours after treatment, the embryos exhibited no evidence of truncation (Figure 1) and exhibited apparently normal eye development (Figure 2). For these embryos, normal development was also observed approximately two days after treatment, by which time most had hatched (Figure 3) and also three days after treatment (Figure 4). No truncation was observed at either time period. The anterior and posterior regions of the embryos developed similarly to that of the control (Figure 5 and Figure 6). There was variation in the amount of melanocytes that could be seen in the embryos, but this variation could not be correlated to the concentrations of retinoic acid with which the embryos were treated (Figure 6). The heads of the embryos treated with concentrations of 10-11 M and 10-10 M also appear to be slightly slanted backwards (Figure 5). These embryos survived until euthanized. The embryos treated with the highest concentration of retinoic acid (10-8 M) were severely affected. These embryos exhibited truncation of the anterior–posterior axis by one day after treatment as well as extreme underdevelopment of the eyes and other anterior structures, such as the brain (Figure 1 and Figure 2 ). These embryos continued development to the second day (Figure 3) but did not hatch. Their yolk sacs were, on average, noticeably larger than those of the control embryos. All embryos treated with the highest concentration of retinoic acid had died by the third day after treatment (Figure 4).