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Look Inside a Developing Fish

Look Inside a Developing Fish
Use a microscope to investigate live embryos—developing eggs—from zebrafish.

Here you can see inside living zebrafish embryos, watch their blood circulate, and compare your own pulse to theirs.

What’s going on?

Zebrafish are widely studied in biology, and for good reason. Not only are they hardy and easy to grow in a lab, their embryos are translucent, so it’s easy to observe how they develop from a single cell into a newly hatched fish.

A fertilized zebrafish egg develops and hatches within 48 hours. It then takes three months for it to reach adulthood.


Zebrafish, 1 hour
1 hour
The fertilized egg has already divided twice, making four cells.
(click image to enlarge)
Zebrafish, 2 hours
2 hours
As the cells continue to divide, they get smaller and smaller (there are now 128 of them).
(click image to enlarge)
Zebrafish, 5 hours
5 hours
The curved line shows where the body will soon take shape.
(click image to enlarge)
Zebrafish, 14 hours
14 hours
The backbone and eyes are beginning to form.
(click image to enlarge)
Zebrafish, 17 hour
17 hours
The brain begins to form. The body continues to grow longer.
(click image to enlarge)
Zebrafish, 24 hours
24 hours
Muscles are taking shape and beginning to twitch.
(click image to enlarge)
Zebrafish, 48 hours
48 hours
The zebrafish is ready to hatch. 
(click image to enlarge)
Going further

Although zebrafish don’t look much like humans, our embryos follow a similar path during early development, so we can study zebrafish to learn about human development.

For example, there are genetic mutations in zebrafish that cause heart defects that are very similar to mutations that cause defects in human hearts. You may encounter some of these mutant zebrafish in this exhibit. They’re easy to spot because their strangely shaped hearts don’t pump blood very well.

External resources

Website: FishScope: Time-Lapse Recordings and Confocal Images Dealing with the Developmental Biology of Fish. © 1995, 1996, 1997, 1998, 1999, and 2000 by Mark Cooper.

Website: The Zebrafish Information Network (ZFIN). © University of Oregon, 1994–2018, Eugene Oregon.

IMLS acknowledgment

This web project was made possible in part by the Institute of Museum and Library Services [MA-30-16-0175-16].

IMLS acknowledgment

This web project was made possible in part by the Institute of Museum and Library Services [MA-30-16-0175-16].

NSF acknowledgment

This exhibit is based upon work supported by the National Science Foundation under grant 1514612. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.