1 UNIT 3 PART 4: EMBRYOLOGICAL DEVELOPMENT In the early stages of development the organism is called an embryo. The basic processes of development are always the same in animals. Development involves cleavage, growth, and differentiation.

2 Fertilization Fertilization occurs when the sperm nucleus fuses with the egg nucleus. A fertilization membrane forms around the egg once a sperm has entered to prevent other sperm from entering.

3 Cleavage After fertilization the zygote starts to divide by a special type of mitosis called cleavage to form an embryo. During cleavage cells divide like in mitosis, but there is no growth between divisions. The cells get smaller with each division.

4 2 & 4 Cell Embryo The first division results in a 2 celled embryo. Each cell in the embryo then divides to form 4 cells. Notice that all the cells are still inside the fertilization membrane.

5 8 & 16 Cell Embryo The cells continue to divide and form 8 and 16 cell stages. Note that all structures are the same size as the original unfertilized egg; the cells produced by cleavage have not grown.

6 Morula After several rounds of cleavage, a solid ball of cells, called a morula, is formed. The cells on the inside have trouble getting water & oxygen and getting rid of wastes.

7 Blastula The cells on the inside of the morula push their way out and form a hollow ball called a blastula. Cells still get smaller with each round of cleavage. The hollow space inside is called the blastocoel. blastocoel

8 Gastrula A gastrula forms when one side of the blastula surface pushes in at a point called the blastopore. This forms a tube that will become the lining of the digestive system. The blastopore is the future anus of the embryo. Blastopore

9 Late Gastrula By the end of the gastrula stage, the cells start to grow between divisions. A third layer of cells begins to grow between the inner and outer layers forming three primary germ layers. The cells in these germ layers will differentiate to become different types of cells. endoderm mesoderm ectoderm

10 DIFFERENTIATION All embryos form from one cell, the zygote, by the process of mitosis. This means that all the embryo’s cells are alike - they all have all the same genes. As development progresses groups of cells will become different and make tissues which form the different organs. These tissues and organs are different because one type of cell uses different genes than another. Example: stomach cells use different genes than eye cells.

11 The Primary Germ Layers Ectoderm: –Cells on the outside –Forms the skin and & nervous system Endoderm: –Cells on the inside lining the tube –Forms the ling of the digestive system Mesoderm: –Cells that form between these layers –Forms the muscles, bones, and organs Mesoderm

12 Review How does the zygote change after fertilization? Label the cleavage stages on the right. What is the blastopore? What is ectoderm & what does it form? What is endoderm & what does it form? What is mesoderm & what does it form?

13 Starfish Development Composite