Response to April 26 Scratchings
General comment: The questions show that in
some cases students are "jumping to conclusions"
concerning identification of the chick and its
parts. This is probably natural, given the very
indistinct appearance of most of the images,
resulting from motion of the chick at this
There are some important lessons from this
- Nature doesn't always give up its secrets to a
casual glance. Sometimes patience and persistence
are required. For example, it may be necessary to
look through the entire database of 200 or so
images each day, to find half a dozen usable ones.
- Things are not always what they seem,
particularly if you look at them with a
preconceived idea of what you will see. Keep an
- You need to consider all the facts in
interpreting your results. For example, if you
see something in one picture that looks like a
foot, you need to ask: "is it in the right
place?"; "can I see it in other pictures that show
the same part of the chick?"; "is it the right
size?". If the answers to any of the questions
is "no", then your original interpretation
("hypothesis") may not be correct. Rather than
contradict student interpretations, I have just
responded "maybe" in cases where the image is too
indistinct for me to make an interpretation.
Another comment: precise descriptions are
important when communicating observations. "White
spot in the middle" is pretty vague. How white
(bright)? How close to the middle ? How big is a
"spot" ? A better description might be: "the
elliptical area at D5, about 3mm long, which is
slightly brighter than its surroundings."
Teutopolis High School:
180 - This picture is very detailed. I could see its beak, head, and feet. The picture gives a perfect view of the fetus.
Teutopolis High School:
In MRI # .00187, Is the chick lying on its back with its feet in the air? MRI # .00188 Where is the chick located? MRI # .00194 What is the spot in the middle of the image?
187 - maybe... 188 - the dark area E-F, 6-7, is the general area of
the chick. This slice is too thin and "snowy" to see much detail.
194 - the larger white spot at D5-E5 appears to be the remains of the
latebra. The little bar at exactly the center is an instrumental
artifact (sort of like dirt on a camera lens). You'll find it in many
Teutopolis High School:
What is the white spot in the center of .00181, .00180, and .00187.
see previous question.
Urbana High School
The image is pretty clear although we aren't sure what it is... We
can't see the developing embryo very well.Mri.00024 is very clear and
it is very easy to identify the embryo, yolk and albumen.
Maybe you meant 124, not 24 ? 124 is an excellent image. 24 is not
Urbana Middle School
We were wondering about several things:
Why is the embryo eye so bog and noticable compared to the rest of the
No one seems to know. We can guess that the eyes are very important
to the chicken, and since they are very complicated organs, they need
extra time to develop. Notice that the head and brain also develop
When does the chicken develop its wings?
According to "Today's News", the wings begin to
appear on day 3. The feathers begin to develop on day 8.
At what stage can you see the embryo's heart beat?
According to "Today's News", the heart starts beating at 42 hours. In
the MRI, the beating heart shows up as a bright spot with a vertical
streak running through it. Similar streaks (motion artifacts) also
appear later for large blood vessels as well. Can you find the heart
in early images? (Days 4-6?)
The book talks about the chicken parts becoming sharp and
"horny". Why does this happen?
Eventually the chick will need to get out of the egg. It will need a
"tool" to do this. This tool is the "egg tooth", and must be at least
as hard as the egg shell. Other parts also need to become hard and
solid, otherwise the chick would just be a "blob" when it emerged from
Why is the MRI image in black and white?
Good question! Actually, the MRI image has no "colors" at all - it is
a radio signal. But in order to make it visible, the computer
artificially colors it. Since a single image conveys only one value
at each point, we choose to assign that value to "brightness". It
could also be assigned to a color, however, to make a "pseudo-color
image". If you have the right software in your computer, you can
change the colors to suit yourself.
The black and white choice has other bases, too. MRI is often done
along with X-rays in a hospital radiology department. The doctors are
used to looking at black and white x-rays, so it was natural to do MRI
in the same way. Often the hard-copies are made on X-ray film, which
is black and white. Finally, if we have two or more images with
different contrast (T1 and T2, for example), we could combine them in
a computer using different colors, to make a much more informative
"false color image".
How often are cells dividing during embryonic development? When
is division the fastest?
I don't know.
C.D.Gregory: April 26, 1996