Why do all vertebrates have very similar forms during development

They tend to show their evolutionary history or phylogeny

Explanation:

Vertebrates are animals that have backbone. During formation of zygote to foetus, they all pass through similar phase and look alike because their genes are similar. They share a particular trait and that is backbone. Generally, vertebrates all come from one family and their morphology is similar during pregnancy and only differ after towards 3/4 end of the pregnancy where each vertebrae have distinct characteristics of its species.