B. 
C. 
D. 
B.
C.
D.
Figure 4. Zebrafish embryos approximately 70 hours after treatment with retinoic acid. The control embryos (A) as well as those in the 10-11 M (B) and 10-10 M (C) solutions continue normal development. All of the embryos in the 10-8 M retinoic acid solution (D) died by this time.