BABCOCK, SK; HURNEY, CA; VAGLIA, JL; TURNER, SD; COGBILL, S; James Madison University; JMU; DePauw Univ.; JMU; JMU: Embryonic Development in the Four-Toed Salamander, Hemidactylium scutatum
We present a staging table of normal development for Hemidactylium scutatum (Plethodontidae: Hemidactyliini), from fertilization to hatching. Terrestrial egg clutches from naturally ovipositing females (n=6) were collected in May 2002 and April 2004. Embryos were maintained at 15oC on moistened filter paper; embryos that were removed from the vitelline membrane were kept in artificial pond water. Observations and photographs of 21 embryos were taken at 12 and 24 hour intervals throughout the 40-45 day embryonic period. Average diameter of single-cell embryos in H. scutatum is 2.41 mm (n=6; range 2.31mm-2.49mm). Development through neurula stages closely resembles development of other salamanders. Thus, we assign early embryonic stage numbers using criteria described for Ambystoma, Eurycea and Gyrinophilus. Stage descriptions are based primarily on external morphology. Consistent with many Amphibia, cleavage in H. scutatum is holoblastic and mesolecithal, with formation of the initial cleavage plane meridional to the animal and vegetal poles. The first sign of gastrulation occurs 3.5 days after oviposition with the appearance of cellular blebbing on the animal surface along with a darkened area of cells at the putative site of invagination. About 1 day later, a crescent-shaped blastopore lip is visible. Consistent with gastrulation in Eurycea, the crescent-shaped lip appears to regress and deepen to form a U-shaped lip that ultimately becomes a slit. Neural groove and folds emerge about 7 to 8 days after oviposition. As neurulation progresses, the neural folds fuse medially with fusion proceeding anteriorly and posteriorly to form the neural tube. Eye, gill, limb, pigmentation, and tail fin development will be presented with discussion of ancestral developmental patterns in Hemidactyliines.
