A birds egg is one cell but when a chick comes out from the birds egg it has many cells . So how does it happen that one cell becomes many?? And its also said that the chick develops from the yolk which is the nucleus . So is it that a chick itself is the nucleus ???

Cell divisions--> basis of growth in living beings. One cell devides into two & two into four and this occurs in geometric fashion. In an egg yolk develops into chick and the rest part provides nourishment to it. Chick itself is not nucleus. Cell divisions occur in that and chick is made up of many cells (multicellular)