THE EFFECTS OF THIOUREA AND PHENYLTHIOUREA UPON THE DEVELOPMENT OF ELEUTHERODACTYLUS RICORDII

¹ Department of Biology, The Catholic University of America, Washington, D. C.

The leptodactylid toad, Eleutherodactylus ricordii planirostris (Cope), is unlike most anurans in that it possesses no aquatic larval stage. Its eggs are laid on land, beneath stones or logs. After about two weeks development within the egg, the young frogs hatch with the adult body form. It was found that eggs immersed in water will develop normally and at the usual rate providing the jelly layers and vitelline membranes are removed. In an attempt to ascertain to what degree the suppression of larval characters and the early assumption of the adult body form are dependent upon the activity of the thyroid gland, developing eggs were raised in solutions of thyroid-inhibiting drugs.

Embryos placed in 0.05 per cent thiourea or 0.005 per cent phenyithiourea on the fifth day of development failed to attain complete differentiation of the limbs and retained the larval tail, so that they were still embryonic in appearance 10 days after the tap-water controls had become complete little frogs. Animals raised in 0.005 per cent thiourea exhibited no retardation in limb development but did retain the larval tail so long as treatment was continued. Treatment with 0.001 per cent thiourea seemed to have no effect on development. Histological study of the thyroid glands of treated and control animals showed marked hyperplasia. hyperemia and reduction in colloid volume in the thyroids of specimens raised in 0.05 per cent thiourea or 0.005 per cent phenylthiourea. Thyroids of animals treated with 0.005 per cent thiourea showed slight hyperplasia and hyperemia but no significant differences in colloid volume as compared with controls. Thyroids of specimens raised in 0.001 per cent thiourea seemed to be unaffected. It appears that, in Eleutherodactylus, the loss of the larval tail and the complete differentiation of the limbs are features which are under thyroid control. On the other hand, the suppression or telescoping of many of the larval features cannot be attributed to thyroid activity since it occurs even under conditions of what seems to be extreme thyroid inhibition.

The embryos raised in 0.005 per cent phenylthiourea showed a rapid loss of pigment which involved not only the skin but also the pigmented coat of the eye. This is probably the result of the well-demonstrated inhibitory effect of this drug upon tyrosinase melanin formation.