First to be developed is ?

Ans. is 'd' i.e., Primitive streako First event in the stage of trilaminar disc stage is formation of primitive streak. o Other three options are formed in the process of neurulation.Trilaminar germ disc stageAge (in days) Developmental events15 Primitive streak appears Definitive yolk sac is formed17 Notochord process appears Hea tube is seen in cardiogenic area Allantoic diveiculum is seen19 Intraembryonic mesoderm is formed Connecting stalk can be distinguished21 Neural groove is seen Head fold begins to form23 Closure of neural tube