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 stage.

There are some important lessons from this exercise:

  1. 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.
  2. Things are not always what they seem, particularly if you look at them with a preconceived idea of what you will see. Keep an open mind.
  3. 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 different images.

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 very good.

Urbana Middle School
We were wondering about several things:
Why is the embryo eye so bog and noticable compared to the rest of the chick?

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 early.

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 the egg.

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

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