The fetus instead develops as a plate - like structure on top of the yolk mass , and only envelopes it at a later stage .
Due to the large size of the yolk , the cell division can not split up the yolk mass .
The fetus instead develops as a plate - like structure on top of the yolk mass , and only envelopes yolk mass at a later stage .
Due to the large size of the yolk , the cell division can not split up the yolk mass .