As I'm fond of mentioning throughout this book, biology has come a long way since Darwin's day, and the field of developmental biology is another good example. Today, scientists are busy trying to figure out exactly how genes work; in Darwin's time, they were struggling just to understand what genes might be. Absent all the molecular techniques available today, it's no surprise that the study of embryology in the mid-1800s involved primarily the examination of embryos from different stages of development.
The following sections look at what early embryologists found — or, more accurately, what they thought they found — and explains what scientists know today.
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