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Biology-Mr.Willets
Lab-Development
With sexual reproduction, once fertilization has occurred, the zygote divides over and over to form the cells that will become the offspring. The pattern of growth varies among different types of organisms but the early stages are similar in most animals.
The growth from one-celled zygote to multicellular organism is called development. Human developmental biology is one of the fastest growing fields in biology as we attempt to learn more about how humans go from one cell to the complex organism that you see in the mirror and as we try to learn why sometimes the process goes wrong.
In today's lab, you will examine some of the early stages of animal development. The books available in lab can help. For all diagrams, USE A PENCIL!!!!
1) Get a slide labelled "Starfish Cleavage". Examine under low power. After gametes fertilize, the zygote divides into two cells. These two then divide into 4 cells etc. The process of repeated cell divisions is called cleavage.
a) Look for a single large cell- This is a zygote or maybe an unfertilized egg. Note its size. What is its approximate diameter in microns? __________ Make a diagram
Once the zygote has divided, it is called an embryo. The term embryo refers to all the early stages of development.
b) Look for two cells together. This is the stage after the zygote. Note that the combined size of the two cells is about the same as the zygote. What is the approximate diameter of the two celled embryo? __________ Make a diagram
c) Look for a 4-cell embryo. What is its approximate diameter? ____________ Make a diagram
d) Look for a 8-cell embryo. What is its approximate diameter? ____________ Make a diagram
As cleavage occurs, what happens to the size of the individual cells? ________________ What (more or less) happens to the overall size of the embryo? ______________________
2) Get a slide labelled "Starfish Dev. Stages". Examine under low power.
a) As cleavage continues to produce more and more cells, eventually the embryo takes the form of a hollow ball of cells. This embryonic stage is called a blastula. Each of the cells in the blastula is very small compared to the cells in the earlier stages seen before. Find a blastula on a slide. Make a diagram of it. Label the diagram: Starfish Blastula.
b) Continued growth of the blastula leads to an embryonic stage called the gastrula. A gastrula has a pocket of cells folded into the ball of cells. The group of cells inside the gastrula is called the endoderm. The outer layer of cells is called the ectoderm. In higher animals, a third layer, the mesoderm, forms. These three layers are called the germ layers and each will eventually form into different parts of the adult body. Find a gastrula and draw it. Label the diagram: Starfish Gastrula. Label the endoderm and ectoderm.
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