Embryology



Embryology is the branch of developmental biology that studies the beginning and early growth of sexually reproducing organisms. This life science specialty focuses primarily on embryogenesis (the formation and development of embryos). In humans, the term embryo is traditionally reserved for the first two months of development. After that point, the term “embryo” is replaced by the term “fetus,” which then applies until birth. At the beginning of the individual’s development, the entity is a single cell. After two months, it has limbs, distinct fingers and toes, internal development, and countless cells. So the term “embryo” applies to an individual throughout a vast range of developmental change.

Since the time Charles Darwin published The Origin of Species, embryology has been used to support common descent. In fact, Darwin referred to embryological homology as the strongest single class of facts that existed to support his theories. A theory later put forth by Ernst Haeckel, known as the biogenetic law, asserted that the evolutionary history of an organism was recapped during embryo development. Although the biogenetic law is now discredited, in recent years, embryology has reemerged as a tool used by evolutionary biologists that attempt to establish phylogenetic relationships by identifying developmental similarities between taxonomic groups. Evolutionary Developmental Biology is a merging of developmental biology and evolutionary biology that is commonly known as "evo-devo".

Embryogenesis
Embryogenesis begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. Approximately 4 days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body.

Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. Therefore, these cells are referred to as pluripotent, that is, they can give rise to many types of cells but not a whole organism. Pluripotent stem cells undergo further specialization into stem cells that are committed to give rise to cells that have a particular function. Examples include blood stem cells that give rise to red blood cells, white blood cells, and platelets, and skin stem cells that give rise to the various types of skin cells. These more specialized stem cells are called multipotent—capable of giving rise to several kinds of cells, tissues, or structures.

Cleavage
After fertilization, the zygote proceeds immediately to the first cleavage (cell division) and subsequent cell divisions follow rapidly. The zygote is a very large cell, but the first waves of rapid cell division occur without increase in cell volume. The result is a closely bound mass of cells each of more typical cell size. At this stage the cells are called blastomeres, and the organism as a whole is called a morula from the time it has 16 blastomeres to the next stage. As it is undergoing this very rapid cell division, the organism is also migrating down the uterine tube toward the uterus.

Vertebrates, such as humans, have bilateral symmetry and therefore polarity in three dimensions (head-tail, or back-front, and left-right). Establishing polarity is one of the most basic manifestations of emerging specialization. But the egg is roughly spherical, and it is not readily apparent how polarity is established. Although it had been shown long ago that the point of sperm entry determines the plane of first cleavage (and thus subsequent ones) in amphibian eggs, mammals were believed until recently to remain spherically symmetrical until later in development. Recent data on mammalian zygotes, however, suggests that the point of sperm entry may similarly determine the cleavage plane. Even the first two cells resulting from the first cleavage may have different propensities, which persist through the next divisions as the progeny of one cell tend to become the body of the offspring and progeny of the other cell become the embryo’s contribution to the placenta and other supporting structures. The word “fate,” however, might be too strong, because the cells of such very early embryos are resilient to perturbations—if one cell is removed, the remaining ones can compensate.

Gastrulation
Gastrulation begins when fluid starts to accumulate between the blastomeres of the morula. The fluid-filled spaces run together, forming a relatively large fluid-filled cavity. At the point when the cavity becomes recognizable, the organism is called a blastocyst. The outer cells of the blastocyst, especially those around the blastocyst cavity, assume a flattened shape. The flattened cells of the exterior blastocyst are the trophoblast. They become the embryo’s contribution to the placenta and other supporting structures. On one side of the blastocyst is a group of cells that project inside into the blastocyst cavity; this is the inner cell mass, or embryoblast, and its progeny form the body of the new offspring.

The cells of the inner cell mass can give rise to progeny differentiating into all the types of cells in the adult body, so they are called pluripotent. They have not usually been described as totipotent because, the inner cell mass having already differentiated from trophoblast, the cells of the inner cell mass were believed to be no longer able to give rise to the cells of the trophoblast. Recent work, however, describes culture conditions under which human embryonic stem cells can differentiate to trophoblast cells. Although the new offspring itself develops only from the inner call mass, the trophoblast is not just passive padding. Its progeny are the essential and specialized connection between the embryonic and maternal systems. Embryonic stem cells can be isolated from the inner cell mass.

Organogenesis
The basic structures and relations of all the major organ systems of the body emerge during the fourth through the eighth weeks of embryonic development. First, the embryo folds in several ways so that the flat linear structure distinguished by neural tube flanked by somites become roughly C-shaped. The effect of this is to bring the regions of the brain, gut, and other internal organs into their familiar anatomical relations. During the fourth week the neural pores, the ends of the neural tube “zipper,” close. First the one at the cranial or head end, which is called the anterior or rostral pore, closes, and later the caudal or tail-ward pore closes. Closure of the neural pores completes the closure of what will become the central nervous system. Also during the fourth week, limb buds become visible, first buds for arms and later for legs. Further, two accumulations of cells along the neural tube become distinguishable: the alar plate and the basal plate. Cells of the alar plate go on to become mostly sensory neurons, while basal plate cells give rise mostly to motor neurons. Already while the neural tube is closing, its walls along the cranial area are thickening to form early brain structure. Cranial nerves, for example the nerves for the eye and for the muscles of the face and jaw, also are beginning to develop at this time. The embryonic brain develops rapidly in both size and structure especially during the fifth week, and the optic cup that will form the retina of the eye becomes visible as well.

Von Baer's law
Several decades before Darwin published the The Origin of Species, a German embryologist Karl Ernst von Baer had shown that the embryos of some vertebrates looked very similar during particular stages of development. This observation became known as von Baer's law. After reading a summary of von Baer's law in 1842, Darwin concluded that embryonic resemblances were a very strong argument in favor of common descent. In The Origin of Species (1859) Darwin stated "Community of embryonic structure reveals community of descent", and in a 1860 letter to Asa Gray stated that "embryology is to me by far the strongest single class of facts in favour of change of forms". Despite Darwin's use of embryology to support his views, Von Baer rejected the theory of common descent and did not regard embryological similarities as evidence for evolution.

Recapitulation theory
The theory of recapitulation was a subsequent attempt to apply embryological similarity in support of Darwinism, which was first put forth in 1866 by Ernst Haeckel. He coined the phrase ontogeny recapitulates phylogeny to summarize the view.


 * Ontogeny is the development of the organism.
 * Phylogeny is the evolutionary history.

Haeckel suggested that gill slits and a tail could be seen during embryo development in organisms that did not possess them as adults. It was asserted that these organism must have possessed structures during their evolutionary history that were lost in adult stages due to natural selection. His theory held that during embryo development an organism progresses through stages in its evolutionary history - a theory that became known as the Biogenetic law.

It is important to note that the gill slits are not slits at all, but merely the folds in the region of the embryo’s throat that are never involved with gas exchange (O2 / CO2), which is accomplished by the umbilical cord. These folds, now called pharyngeal arches, later develop into the outer and middle ear as well as the bones, muscles, nerves, and glands of the neck. The appearance of an embryonic tail is simply due to the rapid growth of the spinal cord causing it to become longer than the body, and forcing the body to curl. This curling contributes to the folds visible in the neck region, in the same way a person develops a double chin when bending the neck forward.

Haeckel's fraud


In an attempt to prove his theory true, Ernst Haeckel committed one of the most infamous examples of scientific fraud. In 1868, he published the series of embryos in the picture at right showing various living things placed side by side in the attempt to give the impression that there was significant similarity among them. The embryos shown in the earliest stages were altered to make them look more similar than they are in actuality. Embryologist Jonathan Wells notes that Haeckel was also very selective about the embryos he chose for his illustration. The four right-hand columns in the diagram at right are all from the same taxonomic subclass Eutheria (Placental mammals). Marsupials and monotremes were excluded, as were cartilaginous fish (sharks) and others. The fraud was exposed only months following the publication of these engravings by L. Rütimeyer, professor of zoology and comparative anatomy at the University of Basel, and corroborated by other contemporaries such as William His Sr, professor of anatomy at the University of Leipzig, who published their own comparisons showing significant differences (picture at left).

In 1997, the full extent of Haeckel's fraud was brought to light when Michael K. Richardson, a lecturer and embryologist at St George’s Hospital Medical School, London, published photos of actual embryos illustrating the discrepancies between Haeckel's popular diagram and genuine embryos. Richardson described the situation in an interview in Science magazine as follows: It looks like it's turning out to be one of the most famous fakes in biology. The effort involved an international team of embryologists who examined and photographed "the external form of embryos from a wide range of vertebrate species, at a stage comparable to that depicted by Haeckel" because up until that time it was apparent that "no one has cited any comparative data in support of the idea". They analyzed 39 different animals including those published by Haeckel and many that were not, such as marsupials, tree-frogs, snakes, and alligators, and they found significance differences with little conservation at the stage of development Haeckel claimed was most similar. In fact, the species in the fraudulent publication are so different that it is assumed the drawings made by Haeckel could not possibly have been done from real specimens.

Richardson summarizes the extent of their findings: We find that embryos at the tailbud stage – thought to correspond to a conserved stage – show variations in form due to allometry, heterochrony, and differences in body plan and somite number. These variations foreshadow important differences in adult body form. Contrary to recent claims that all vertebrate embryos pass through a stage when they are the same size, we find a greater than 10-fold variation in greatest length at the tailbud stage. Our survey seriously undermines the credibility of Haeckel’s drawings, which depict not a conserved stage for vertebrates, but a stylised amniote embryo.

In an interview for the London Times, Richardson is quoted as saying: This is one of the worst cases of scientific fraud. It’s shocking to find that somebody one thought was a great scientist was deliberately misleading. ... What he did was to take a human embryo and copy it, pretending that the salamander and the pig and all the others looked the same at the same stage of development. They don’t … These are fakes.

Textbook use
Despite the early exposure of this fraud by his contemporaries and following the almost universal recognition by the scientific community, Haeckel’s embryo comparisons continue to appeared in textbooks in support of common descent. Examples of recent use include the 2004 edition of Biology: The Unity and Diversity of Life by Starr and Taggart. , and the 1992 edition of Biology, a college level introductory biology text that states: "In many cases the evolutionary history of an organism can be seen to unfold during its development, with the embryo exhibiting characteristics of the embryos of its ancestors. For example, early in their development, human embryos possess gill slits like a fish..."

In 2000, the eminent evolutionist Stephen Jay Gould calls the continued use of these embryos in textbooks atrocious: We do, I think, have the right to be both astonished and ashamed by the century of mindless recycling that has led to the persistence of these drawings in a large number, if not a majority, of modern textbooks. (Stephen Jay Gould)

Homology

 * Main Article: Homology

Similarities that exist between different organisms are cited by evolutionists as one of the most compelling lines of evidence for their theory. Homologous features are shared design concepts that are generally assumed by evolutionists to be the result of a shared ancestry and therefore offering support for the Darwinian theory of common descent. In The Origin of Species, Darwin referred to the similar bones in the same relative positions in the hand of a man and several other mammals as specifically what the theory of common descent would expect. The 2008 edition of Prentice Hall Biology by Miller and Levine illustrates that this concept is commonly held wherein it states: In their early stages of development, chickens, turtles, and rats look similar, providing evidence that they shared a common ancestry.

Much like gene comparisons, the similarities that are evident during embryo development (von Baer's law) are assumed outright to be due to phylogenetic relationships and therefore homologous. Furthermore, embryo homology is considered to be such powerful evidence of common ancestry that patterns during embryogenesis are used to determine if shared morphological structures are truly homologous (due to shared ancestry) or merely analogous (due to convergent evolution). Miller and Levine define homology in such terms: structures that have different mature forms in different organisms but develop from the same embryonic tissues.

The reemerging of the early 19th century "Von Baer's law" has stimulated a new biological discipline called Evolutionary Developmental Biology, which is a collaboration of Evolutionary Biology and Developmental Biology, popularly known as Evo-devo. This area of scientific inquiry draw from the beliefs of Darwinism that all organisms have descended from the same ancestor, and generally assumes that embryonic developmental mechanisms should be shared (conserved). Drawing from these assumptions, evolutionary developmental biologists compare the developmental processes of different animals and plants in an attempt to determine the ancestral relationship between organisms.

Developmental biologist William Ballard a well known elder developmental biologist at Dartmouth College comments on the impact of von Baer's law on modern embryology. Thus, the energy of investigators and particularly students is diverted into the essentially fruitless 19th century activity of bending the facts of nature to support second-rate generalities of no predictive value. Though enthusiasm for Haeckel's (1900) recapitulation `law' died out, unfortunately the popularity of Von Baer's `laws' of 1828 was renewed. In order to defend the latter's descriptive statements that general characters appear before special characters as an egg develops and that the less general and finally the specific characters trail along later, we have to decide intuitively that certain characters are of `morphological significance' and others are not. When referring to vertebrates, we have to use words like blastula and gastrula in such a way as to imply that things that are vastly different from each other are really very much the same.

Differences among organisms have traditionally been assumed to become more significant as development proceeds, and therefore earlier embryonic stages should resemble one another more than latter stages. This view can be found presented in popular biology textbooks. Although these assumptions have persisted into modern times, it has been recognized by embryologists since Darwin that this doctrine of developmental biology "is not in accordance with the facts of development", according to British embryologist Adam Sedgwick (1894) who also stated: there is no stage of development in which the unaided eye would fail to distinguish between them with ease". Modern embryologists confirm this as well, and have further challenged the erroneous nature of these assumptions. Recent work has revealed that embryos are not more similar during the earlier stages of cleavage and gastrulation, but in fact, are most alike at the later stage illustrated by Haeckel and recently photographed by Richardson. In the gastrulation stage, fish show dramatic differences when compared to amphibians, and neither are similar to reptiles, birds, or mammals. Organisms begin life very dissimilar, then become somewhat alike during embryo development, then diverge again as adults. Evo-devos (evolutionary developmental biologists) refer to this pattern as the developmental hourglass. William Ballard adds that only by semantic tricks and subjective selection of evidence can we claim that “gastrulas” of shark, salmon, frog, and bird are more alike than their adults.

Ballard continues: In fact, the most obvious structural characteristics of either the eggs or the cleavage stages of a shark, a salmon, a frog, a bird, or a mammal are unique each to its own class, not generally shared. We would not consider them very much alike unless we had been taught so at a very early age. Very few vertebrates pass through a stage which can strictly be called a blastula. The embryo in its period of most active morphogenetic movements is usually called a gastrula, but as all agree this word has no morphologic meaning anymore. Each class of vertebrates (in mammals we might almost say each particular order) develops and then loses its own set of temporary structures-like the parade ground `formations of maneuver' during this period.

Another significant challenge for evo-devo is the recognition that organs and structures develop from different groups of embryo cells in organisms that are believed to share a common ancestry. Fundamental structures, like the alimentary canal, kidney, or forelimb are commonly found to form from different embryological tissues. The alimentary canal (digestive tract) is formed from the roof of the embryonic gut cavity in sharks; whereas in frogs it is formed from the gut roof and floor; and in birds and reptiles it is derived from the lower layer of the embryonic disc or blastoderm. Vertebrate forelimbs, which were used as an example of homology by Darwin and can be found in hundreds of textbooks as proof for evolution, also develop from different body segments in different species. Rather than being proof of Darwinism, recent embryological studies have shown that forelimbs are not developmentally homologous. The forelimbs in the newt develop from embryo trunk segments 2 through 5; in the lizard they develop from segments 6 to 9; in humans they develop from segments 13 through 18. The development of the vertebrate kidney also conflicts with the view of homology. Evolutionist Michael Denton notes these conflicts in Evolution: A Theory in Crisis.

In fish and amphibia the kidney is derived directly from an embryonic organ known as the mesonephros, while in reptiles and mammals the mesonephros degenerates towards the end of embryonic life and plays no role in the formation of the adult kidney, which is formed instead from a discrete spherical mass of mesodermal tissue, the metanephros, which develops quite independently from the mesonephros.

Studies of the genes governing development have added even further difficulties for evo-devo. According to the views of neodarwinists, homologous features are produced by genes that have been passed down by common ancestors. Because similar genes are unlikely to have originated independently through random mutation, sequence similarity between organisms indicates they share an ancestral relationship (they are homologous). Features produced by similar sequences are then also inferred to be phylogenetically homologous. For decades it has been recognized that homologous organs were frequently produced by different genes, but more recently, evo-devo biologists discovered that certain developmental genes were highly conserved (similar) in radically different organisms. These homeotic genes are short sequences that act as regulatory switches affecting the development of structures such as eyes and legs. When mutated, homeotic genes can transform one body part into another, such as an antenna into a leg. In organisms as vastly different as mammals and fruit flies, these genes are so similar that the mouse gene can regulate the development of the compound eye of the fruit fly. In such examples, the conflict with Darwinian assumptions lies in the fact that non-homologous structures are produced (or at least regulated) by supposedly homologous genes. As Jonathan Wells states very poignantly: homology, whether defined morphologically or phylogenetically, cannot be attributed to similar developmental pathways any more than it can be attributed to similar genes.

The famous embryologist Sir Gavin de Beer challenged his colleagues with the following question which remains unanswered to this day. But if it is true that through the genetic code, genes code for enzymes that synthesize proteins which are responsible (in a manner still unknown in embryology) for the differentiation of the various parts in their normal manner, what mechanism can it be that results in the production of homologous organs, the same "patterns", in spite of their not being controlled by the same genes? I asked this question in 1938, and it has not been answered.

Worldviews
While Darwin was convinced that embryological homology was the strongest single class of facts to support his views. Modern embryology has instead challenged the Darwinian theory of common descent by illustrating that seemingly homologous structures, such as the mammalian forelimb, developed along different pathways, and are therefore not homologous at all.

While the Darwinists see these similarities as evidence of common descent, the creationist belief system causes us to generally conclude that shared structures, processes, or genes are merely reflections that they were designed by the same creator. Rather than being evidence of shared ancestry, homologies are examples of brilliant and well functioning designs that have been applied to multiple organisms, much in the same way as human designers apply concepts.