Reading Assessment: Chapter 6 Your Inner Fish

What the reader learned in the first couple pages was every animal goes through the same stages of development as embryos. Every animal originated through one of the three layers of tissues called germ layers. During fertilization, a sperm and egg fuse together and the egg divides and forms a ball. For humans, the cell body divides four times (16 cells). The cell ball is called a blastocyst. The blastocyst implants itself in the moms uterus halfway and the baby's body forms from the top part of the blastocyst. When the cells divide, the baby becomes a "tube". It is called a "tube within a tube structure". The outer layer of the tube is the ectoderm (skin and nervous system), the middle later is the mesoderm (forms tissue in between guts and skin) and the inner layer is the endoderm (inner structure of the body, digestive tract and glands). 

Hans Spemann, an embryologist found that some cells can form a whole new individual on its own. He used newt eggs and pinched one side of an egg from its other side and the embryo formed twins. Hilde Mangold transplanted a tissue from one embryo to another embryo specie. She put the patch where the three germ layers are. She discovered a small patch of tissue, called an organizer, that was able to tell other cells what to do. All mammals, birds, amphibians and fish have organizers and it is possible to swap one species organizer for another. 

Flies have crazy mutations because they have an error in their DNA. Genes are stretches of DNA that are on the chromosome. 8 genes called hox genes are responsible for the development. Versions of the hox genes appear in every animal with a body. Many genes interact in the organizer to organize the body plan.