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 19th century by Ernst Haeckel. This is totally unjustified.
The difference between Von Baer and Haeckel
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
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.
Annelids and Molluscs
"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.
Mammal embryo's do not have gill slits. They have gill pouches. They are related homologically to the gill slits in fishes, which means they are in the same part of the embryo and develop from the same part of the bastulae.In fish, of course, these develop into gills. In jawed fish, they also develop into the jaws. In mammals, they do not develop into jaw, ear and throat organs, as can be seen in this quote from the AiG X Nilo files (Vol 1, no 2) "In fact, the creases in the human embryo which Haeckel referred to as "gill slits" have no connection with breathing, but develop into ear and jaw areas." This is very revealing because from fossil studies we know that mammalian ears evolved from reptilian jaws and that reptilian jaws evolved from fish jaws, which evolved from fish gills! The prescence of gill pouches in human embryo's give an insight into our ancestors.
It is not just the shape of the embryo that shows its ancestry. It is way that the embryo develops.
Cleavage is the first few splits of the egg that form the blastuale. Echinoderms (such as starfish and sea lilies) have the same pattern of cleavage as chordates. This is exactly what we would expect, as from a biochemical point of view, we are closer to starfish than insects. Both echinoderms and chordates have a radial cleavage:
Annelids, molluscs and arthropods use a spiral cleavage:
Spiral cleavage is simpler, and is closer to what thermodynamics would expect for bubbles forming naturally in that way. What is also interesting is that if part of a spirally cleaved embryo is removed, it forms only part of the embryo. If animals that show radial cleavage, which is more sophisticated from a thermodynamic point of view, have a part taken from them, both sections can form whole organisms.
The next diagram shows how the vertebrate animal gastrulation (formation of the gut) occurs:
Once the bastulae has developed then the rest of the organs can too. The following diagram is the way that these develop in amphioxus. Once again, this can be used to show the development of any vertebrate.
It is possible by examining the embryos of disparate classes of animals to calculate the possible common ancestor. In itself this is not enough to confirm the theory of evolution, but if the evidence from this supports the evidence from other areas, which is it, then the case is very strong.
Man and the Vertebrates Vol. 1. A S Romer Pelican Books 1954
Biology a functional approach 4th edition M B V Roberts Nelson 1986
The Theory of Evolution J M Smith Pelican books 1958