It's a sack and not an egg but the primary food source, the only one, is the placenta and the nutrients go via the umbilical cord.
The yolk nourishes the developing embryo.
yolk
An egg yolk is the part of an egg which serves as the food source for the developing embryo inside.
biomass
The egg yolk is the main source of food for a developing embryo, and the albumin (egg white) supplies other nutrients as well as water to prevent the embryo from drying out.
The egg yolk provides nutrients to the chick embryo when it is developing. It is a major source of protein, B12, and phosphorus for humans. It is also a good source of selenium.
The egg white provides a semi liquid equivalent of the human amniotic fluid environment for the growing chick. The embryo chick starts as a tiny speck attached to the yolk. The yolk is the protein that the embryo uses to grow.
It is a primary source.
It's a sack and not an egg but the primary food source, the only one, is the placenta and the nutrients go via the umbilical cord.
what is the source of nourishment for developing fish embryos?
A picture can indeed be a primary source.
it is a primary source
They act as a food source for the embryo