Developmental Biology of Frogs

This lesson was developed by Dr. Cindy Timmerman, who has a PhD in developmental biology from the University of Florida

You can find
additional resources for this lesson by clicking on these links:

Glossary

Tadpole Tales on student website

Kansas State and National Standards



Overview:

Presentation to students. Students will learn about frogs, and the developmental stages that frogs go through from egg to embryo to tadpole and finally to the adult form. Basic frog biology, including anatomy and ecology, will be presented.

    
Grades:  5-7 and 8-12

Kansas State Standards can be found in a Google Document that we provide that contains both the Kansas State and National Standards.
 
Objective:  

  • Students will understand the importance of frogs and other amphibians.
  • Students will recognize the different developmental stages of frogs.
  • Students will learn about complex life cycles.


Materials:  A computer with access to the internet and Microsoft Powerpoint.

Methods: We recognize that it may not be possible to take students on a field trip to find frogs in nature, or to rear them in your classroom, so we provide a lesson that can be done indoors using a computer. In the best possible scenario this lesson would supplement a field trip to a local pond during frog breeding season. The computer lesson consists of students viewing the Tadpole Tales slideshow and reading the story of the two frogs that were raised from eggs to adults and then learning the different developmental stages that the frogs passed through. We provide a YouTube video and slideshows on this page to supplement the story with scientific terminology and concepts. There is an exercise in the attachment section at the bottom of this page that can be done by students using PowerPoint, instructions are included in the attachment.

Instructor will: Present in a lecture format an explanation of the basic anatomy and developmental biology of frogs based on the photographs, outline and illustrations provided in this lesson.

Students will: Students will demonstrate an understanding of the changes that frogs undergo as they develop. Concepts such as metamorphosis from aquatic tadpole to semi-terrestrial adult will help students to learn basic embryology as well as the ecological connectedness between aquatic and terrestrial habitats.

Evaluation: Knowledge of basic anatomy will be demonstrated by having students work through the "puzzles" presented in the PowerPoint in the attachment section at the bottom of the page.

Students will demonstrate: An understanding of how the differences between tadpoles and adult frogs lead to different lifestyles and ecology, and how that affects the river and other habitats.

Lecture Notes

The goal of this lesson is to familiarize the student with the awesomeness that is frog embryonic development.

A frog begins life floating in water, encased in a jelly-like egg coating.The heavy yolk fuels embryonic development and orients the egg so that the black spot is near the top of the egg, helping to warm the egg in the sun. After fertilization, the egg begins the work of building the frog. The stages of frog embryonic development are significant because they are virtually the same for all chordates, including humans.

The zygote undergoes 4 basic embryonic stages of development:
  • morula
  • blastula
  • gastrula
  • neurula
The fertilized egg is essentially one large cell, but as time goes on, it divides several times until it is a ball of several equal-sized cells. It looks sort of like a peeled orange. This is the morula stage.

 
 
 



All of these cells appear the same and are totipotent, but as they multiply further, within 7 hours, they start to become specialized and migrate. Depending on the chemical and electrical signals from the environment, and neighboring, embryonic cells start to migrate and specialize into specific germ line. The blastula is created when these cells arrange to form a fluid-filled ball. When the blastula begins to flatten and form a hollow cup-shape, it is called a gastrula. It looks a bit like a deflated basketball.The edges of the cup start to curve toward each other to form a new hollow center called the archenteron. This chamber will eventually become the gut of the animal. The tiny pore leading to the archenteron chamber is called the blastopore. This will eventually become the anus. It is during this gastrula stage that the three germ cell layers are created: endoderm, mesoderm and ectoderm. These 3 germ cell lines give rise to specific organs and tissues within the body that are the same for all chordates.

Although there are other kinds of animals whose development is based on two germ cell types, all chordates have a body plan that is based on these three basic cell types, and therefore they are called triploblastic.


Now that we have these three germ cell types, neurulation and organogenesis can begin.The outermost layer of cells (ectoderm) of the gastrula begins to form two parallel ridges that rise up and converge on each other. The top of these two ridges curve to meet each other and the result is a dorsal hollow tube that will eventually become the brain and notochord. This whole process takes less than 48 hours.

After 3 to 7 days (depending on the water temperature), the zygote has most of its basic body plan laid down and is ready to hatch. In frogs, they don’t really hatch, as much as the jelly-like covering begins to break down when the zygote begins to wiggle about, trying out its new muscles and nerves.

The hatchling’s first meal may be the remnants of this egg coating. Ectoderm eventually gives rise to the skin. Mesoderm will form the notochord, muscles, skeleton, kidneys, gonads and most of the circulatory system. The endoderm will become the lining of the digestive tract, liver, pancreas and lungs.



Many species of frogs are being affected by water pollution, and in some areas there are such severe population crashes that species may be in danger of extinction. You can expand on this lesson with a discussion of abnormalities in frog development caused by (1) increases in parasites in nutrient enriched waterways, (2) from agricultural chemicals, and (3) from pharmaceuticals in livestock and human waste. Such a discussion will provide a bridge between Tadpole Tales and Caring for the Kaw, which explores the impact of stormwater runoff on water pollution.

Here are a few resources that can be used for discussion on this topic:

Frog deformities linked to farm pollution

Pesticide Atrazine Can Turn Male Frogs Into Females

Sterility in Frogs Caused by Environmental Pharmaceutical Progestogens, Study Finds

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Dr. Cynthia Annett,
Mar 20, 2012, 8:42 PM
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Dr. Cynthia Annett,
Mar 20, 2012, 8:39 PM