Animal Writes
26 May 1999 Issue
What the Heck is an Egg Yolk?

A question from Alexis D'Angelo, who is a 7th-grader in Ms. Arlene Hicks' class
at Pierce Middle School, Grosse Pointe, Michigan. Alexis' original question
concerned the fact that she had heard that the yolk of an egg was a cell -- if so,
where was the nucleus -- where were the ribosomes?

The egg of a bird is what we are talking about here. But, the female of every
living thing that has females and males, makes eggs or something very much
like an egg.

In the case of animals, the egg consists of a single ovum (cell) which contains
one-half of the normal genetic material -- one-half of the chromosomes. For
human females after puberty, about every 28 days one egg is released from
one of the two ovaries. Usually, alternate ovaries are used every month. As
the egg travels down the fallopian tube, if it does not interact with a sperm, it
will eventually be shed along with the lining prepared in the uterus each month.
This lining has a very good blood supply and would provide the nutrients
necessary for the development of the embryo if the egg were fertilized. If the
egg is not fertilized, this shedding of the egg and lining causes little blood
vessels to open, which results in bleeding until the blood vessels heal (usually
very rapidly).

If the egg encounters a sperm within the fallopian tube, the egg may become
fertilized by this sperm cell (which also contains one-half of the normal
chromosomes). These chromosomes are introduced into the cytoplasm of the
egg cell, and the nucleus of the sperm fuses with the nucleus of the egg and
now there is a new nucleus which contains the complete number of chromo-
somes for a normal cell for that organism, and the development of the embryo.
The egg continues its travel to the uterus, where it becomes implanted within
the uterine wall (the lining I talked about) and begins to receive nourishment
from the bloodstream of the mother. This fertilization process and implantation
of the fertilized egg within the uterus initiates a complex series of reactions
which leads to cell division - from one, to two, to four, to eight, etc., and to a
complex series of reactions which leads to differentiation (development) of
these cells along a specific timeline (temporal) which ultimately leads to
individual organ development and finally the birth of a complete, living organism.

In the case of birds, this ovum (egg) is certainly different-looking. The female
chicken produces a "single cell" (has a nucleus) with one-half the chromo-
somes inside, enclosed within a membrane "bag" filled with what we call "yolk"
(nutrients, food). On the outside of this yolk sac is a bunch of protein called
albumin, then a thin membrane completely surrounding the albumin, and finally
the hard shell of the egg (mostly calcium). So, this egg is one, single, entire
cell -- pretty large, huh? If you look inside an egg, you will see two little twisted
chord-like things at each end of the yolk sac... these little connections, each
called a "chalazas" connect the yolk sac to the membrane which lies just
beneath the shell. These are the little white "things" you see when you break
an egg into a bowl... the ones which are really hard to get out of there... <grin>

The chicken releases this egg inside its body, similar to how humans release
an egg... but -- this egg is deposited on the outside of the chicken whether or
not it is fertilized. If it is fertilized by a sperm from a rooster, the nucleus of
the sperm will fuse with the nucleus of inside the egg to form a new nucleus
which now contains all of the information for development of a chicken. The
formation of the hard shell will occur -- again, whether or not the egg is
fertilized -- takes time to form the shell, so the sperm has to fertilize the egg
before the shell completely forms. After the egg is deposited on the outside of
the chicken, with warmth (chicken sitting on it) the fertilized cell within the egg
continues to divide and a baby chicken will grow inside, using the nutrients
which are coming from the yolk. The nutrients will be used until finally, the
baby chicken will start to peck (an automatic differentiation response - kind of
like breathing when a human baby is born) and this pecking will break the
shell, and the baby chicken will crawl out.... snakes do this too, and fish do
this.. although of course, neither of these "peck" anything... but - they do force
their way out of the egg. Snake and fish egg coverings are soft -- don't need a
sharp beak to break out... Actually, when little snakes are born, most of the
leathery egg covering is gone. Other reptiles (like turtles) lay complete eggs,
and the little turtles have to work their way out.

-- Anon E. Mous

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