The relationship between the presence of cysteine lyase in the yolk sac endoderm and the disposition of the area vasculosa in the chicken blastoderm.

Two series of experiments were carried out to test the possible relationship between the presence of cysteine lyase and area vasculosa in the chicken blastoderm. In the first series, the localization of cysteine lyase was compared with the disposition of the area vasculosa at the successive stages of gastrulation and neurulation. In definitive streak stage chicken blastoderms, the enzyme first appeared laterally at the area pellucida-area opaca border. At older stages, this positivity extended rostrally and only finally did it appear in the caudal area. Based on these in toto observations, the relationship with area vasculosa, the distribution of which has been studied by Settle ('54), seems only incidental. In the second series, intermediate streak stage blastoderms were transversely dissected so that the rostral part did not contain any mesodermal component. The presence of cysteine lyase in this rostral part after 15 hr of culture suggests that the appearance of cysteine lyase in the yolk sac endoderm is not dependent upon the presence of area vasculosa. Moreover, the results suggest that the yolk sac endoderm of the chicken blastoderm is determined to produce cysteine lyase from the intermediate streak stage on.