Differences in Injection of X. laevis and X. tropicalis Differences in injection of X. laevis and X. tropicalis. Includes: X. tropicalis lays eggs about 4 hours after a boost of hCG; In vitro fertilization is not nearly as efficient in trops compared to laevis; X. tropicalis embryos are much softer than X. laevis embryos; X. tropicalis embryos whose jelly coats are removed by cysteine have a loose sticky vitelline membrane; X. tropicalis do not have a "summer slump".