Vertebrates

Animals are split into two groups based on whether or not they have a backbone. Vertebrates are animals with a backbone and a skeleton with muscles attached to it. Invertebrates have a soft body and in some cases is supported and protected by an exoskeleton or shell. Vertebrates include fish, amphibians, reptiles, birds, and mammals.

Development

Some vertebrates develop externally in water (fish and amphibians), some develop externally on land (reptiles and birds), and some vertebrates develop internally (mammals). The hard shelled egg made it possible for animals to develop on land. Vertebrate embryos in the early stages of development look remarkably similar. This is because the genes that control early development have a high rate of conservation. The similarity between animals in early development and the presence of vestigial features like gills in mammal embryos was one of the observations Darwin used as support for the evolution of life on Earth.

Zebrafish Development

Zebrafish are used to study development because of their relatively rapid life cycles and their clear embryos. Zebrafish begin as a fertilized single cell (zygote) above a yolk. This cell divides many times and the cells begin to migrate over the yolk. The embryo develops on the edge of the yolk and eventually it will break out of it’s outer membrane and become free swimming. You can see a video of early development here. The models below represent early stages of zebrafish development. The Mandarin orange is the yolk. Panel 1 is the cleavage period with the blueberries representing the 4 cell stage, panel 2 is the blastula period and the sprinkles on the chocolate represent the cells, panel 3 represents the gastrula period with the peel representing the cells migrating over the yolk, and panel 4 represents the segmentation period.