Very early stage of chick embryo cultivated in the medium containing DLtryptophan by beaker method has been investigated in vitro morphologically using electron microscopy at cellular level and found that the development of tryptophan treated chick embryos corresponding to 18∼66 hrs incubation was impaired and york granule degradation was significantly delayed. It was also found that DNA, RNA and protein biosynthesis of tryptophan treated chick embryo was greatly lowered than those of control group. Conversion of L-tryptophan into serotonin was traced using 14C-L-tryptophan and found that 13.8cA of added radioactivity was recovered from serotonin formed during 18 hrs incubation and the amounts of serotonin formed were depend upon added amount of tryptophan in e99 yolk. It seemed that the serotonin formed from external tryptophan might inhibit the degradation of yolk granule by feedback mechanism, resulting in malformation of chick embryogenesis.