Embryology

=Early Embryos and Later Embryos.= = = Early Embryos have significant differences. = = = = Later Embryos show basic vertebrate body plan above. I just like this picture below. Zygote dividing.

Figure shows basic differences in very early development. More patterns of cleavage in organisms below.

The Earliest Stages Both Darwin's and Haeckel's comparisons left out the earliest stages of development.* Whether this omission was intentional is a matter of some debate. Haeckel knew about the early differences, because he drew them. **Eggs, cleavage stages, and gastrula stage embryos**

Some organisms for mouth first and others form anus first.

Similarities in Early Development on page 235 in our textbook is deceptive. There are a lot of types of embryo development which are more different at the start and converge. This converging before diverging probably occurs because the organisms all have a backbone and four limbs. The later basic embryo shapes which are similar then diverge into the animal groups.

Haeckel’s deception was exposed by the embryologist of his day. Haeckel’s deceptive pictures none the less existed in textbooks for a 100 years. Our textbook still presents his idea, but leaves pictures out. Here are non-deceptive pictures, some of them by Haeckel himself.



Anus first or mouth first