Abstract
While investigating the effect of several substances which may influence the growth of rickettsiae in the yolk sac of the chick embryo, some effects on the embryo itself have been observed, and these have been followed up for thiourea.
Thiourea and related substances have been shown to affect the thyroid function in adult animals, viz., rat, 1 , 2 mouse and dog, 1 rabbit, 3 and chick. 4 Thiourea has also been found to inhibit metamorphosis in tadpoles 5 - 7 and cleavage of sea urchin eggs. 8
Thiourea is introduced into the yolk sac of Leghorn embryos according to the technic described by Cox. 9 Aseptic precautions are strictly adhered to throughout the whole procedure. Fertile eggs incubated at 39° C for 7-17 days are used. The shell covering the air sac is washed with 5% phenol and the injections are made into the yolk through a needle-sized opening in the air sac by means of a hypodermic syringe and a 22-gauge needle. The hole is then sealed with paraffin and the eggs are returned to the incubator. Amounts of thiourea used were 0.3 to 3 mg per egg. In order to avoid mechanical damage to the embryo, the fluid introduced should not exceed 0.5 ml; a similar volume of distilled water serves as control. Eggs were daily inspected by candling and dead ones were discarded.
Results. While the controls hatched after 21 days (on very few occasions after 20 or 22 days), hatching of embryos treated with thiourea was retarded up to 10 additional days. The retardation seemed to depend on 2 factors, (a) age of the embryo at the time of injection; (b) concentration of thiourea. For example, 1 mg thiourea delayed hatching by only 1 day in 17-day egg embryos, while the same amount applied to younger embryos (10 days old) caused a delay of 3 days. Two mg of thiourea retarded hatching by 3-7 days, the effect again depending on age. Though hatching time can be extended up to the 30th day by still higher amounts of thiourea (3 mg), hatchability is much impaired being equivalent to only about 5-10% of the controls. In addition, the delay in hatching is associated with other signs of inhibition.
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