Embryology is the study of the development of animals after fertilisation takes place. The embryo needs to pass through three stages, cleavage, where the single fertilised egg divides into many cells, gastrulation, or the formation of the gut which is where the embryo forms different layers and organogeny, the formation of the organs.

The study of developmental biology as it pertains to evolution was really begun before "Origin of Species" was even written. In 1828, a noted embryologist called Karl Ernst Von Baer had two embryos preserved in alcohol, which he forgot to label. He was rather annoyed to find, when he went to study them, that he could not determine whether they were lizards, birds, or even mammals.

He was actually rather disturbed by his findings, considering that evolution hadn't been realised yet. He didn't quite understand why the embryos not only looked identical in early development, but developed according to the same pattern. And we now know that organisms more closely related evolutionarily diverge from each other (in embryonic likeness) at much later points. For example, if you take a fish, a salamander, a tortoise, a chick, a hog, a calf, a rabbit, a monkey, and a human then The fish and salamander will diverge first into recognisable types. The others are all impossible to tell apart at this point. Then the tortoise, then the chick, then the hog and calf, then the rabbit, then the monkey and human at a much later point.

Embryology has a bad press amongst Creationists due to the work of one fraud commited in the 19^th century by Ernst Haeckel. This is totally unjustified.

Von Baer noted that the embryos of creatures resemble each other more than the adults do, Haeckel later said that embryo's show the evolutionary pathway of descent by resembling the adult of each creature in its family tree. The difference between these two views can be seen by considering the gill pouches in a human, reptile or bird embryo. Haeckel said that the presence of these gills showed that the embryo was going through a 'fish' stage. Von Baer said that the gill pouches show a relationship between a terrestrial vertebrate embryo and a fish embryo.

From study into embryology, Von Baer developed four laws that habe been named after him

1. The general features of a large group of animals appear earlier in the embryo than do specialised features.
   All developing vertebrates appear slightly after gastrulation, and it is only in later development that features of class, order, and species emerge. All vertebrate embryos have gill arches, notochords, spinal cords, and prenephrotic kidneys.
2. Less general characteristics are developed from the more general, until finally the most specialised appear.
   Still developing vertebrates have the same type of skin. Only much later does the skin develop into claws, scales, hair, feathers, nails, what have you. Similarly, the development of the limb is essentially the same in all vertebrates.
3. Each embryo of a given species, instead of passing through the adult stages of other animals, departs more and more from them.
   More clearly, this states that the visceral clefts (for example) of embryonic mammals and birds do not resemble the gills of adult fishes, but rather the visceral clefts of embryonic fishes and other embryonic vertebrates. Mammals later convert these structures to eustachian tubes and the like, but they all start out the same.
4. Therefore, the embryo of a higher animal is never like a lower animal, but only like its lower embryo.
   

Haeckel knew there were problems with his theory. The problem can be seen by examining the evolution of the amnion and allantois (see diagram below). Both of these were necessary for the reptilian egg to survive out of water and so enabled vertebrates to live on the land. Haeckel acknowledged that these were evidence against his law of recapitulation, because they are membranes that could not possibly be present in an adult form. These are present in the embryo of birds and mammals, suggesting that they evolved from a common ancestor that was a reptile. This fits Von Baers theory because these are features in the common ancestral embryo.

Now that we can ignore the work of Haeckel, but are still aware of the importance of embryology to evolutionary studies, we can loook at a couple of examples that highlight the way that embryology confirms the theory of evolution.

"In both cases the zygote cleaves to give rise to strikingly familiar bastulae, in both of which a group of small micromeres are arranged in a characteristic pattern above a smaller number of larger macromeres; the method of gastrulation is the same in both, as is the formation of the mesoderm and coelomic cavities. The mesoderm affords the most impressive similarity: It is derived entirely from the proliferation of a single cell; precisely the same one in both cases. If this is not convincing, a glance at the larva should clinch the matter. Both animals share a so-called trochophore larva, a little creature with a curved gut, a characteristic girdle of cilia and a number of other diagnostic features." "Biology a functional approach - Fourth Edition" M B V Roberts Nelson 1986

This is a rather technical description of how the embryo of annelids and molluscs form, but in essence it is saying that even though the two groups of animals, annelids (worms) and molluscs (shelled marine creatures, octopi and squid) are very different, the embryos are indistinguishable. This is a powerful piece of evidence that they are related in some way, and the most obvious way is that they are descended from the same ancestor which had an embryo similar to this. The fact that the mesoderm in these totally different animals come from the same cell of their respective embryo's really points towards this conclusion.

When the quote above talks about how the larvae are the same, it is talking about the fact that the diagram below can be used to show the features of a worms embryo or a molluscs. The fact that an embryo of this

Above is a picture of a terrestrial vertebrate. If anybody wants to copy this diagram for a project, then you are free to do so. Remember to relable the diagram to show whether you are talking about a reptile embryo, a bird embryo, a mammal embryo or a human embryo. The same diagram will work in every case. The fact that the same diagram will work highlights the striking similarity between the three embryos of totally different classes of animal.