In the recapitulation in man we can detect traces of hydras, notochords, cartilaginous fish, bony fish, amphibians, reptiles, mammals, primates, apes, and finally humans. What took six hundred million years of evolution to develop in the case of humans takes place inside the womb in nine months.

The early phases of development cover longer phases of evolution than the later stages. Five hundred eighty million years of evolution is covered in the first six and a half months. Less than twenty million years is then covered in the remaining two and a half months before partition (birth). Each fetal hereditary step has undergone first a test of survival in the living world before it was submerged into the fetal stage in the egg or womb where it exists in a specially protected period of development.

While in the special environment of the womb, the developing fetus may sacrifice anatomical features from its hereditary past that are not essential for survival. Through evolution, negative mutations have erased parts that are not needed for development. For instance, a mutation that destroys the scaly integument of the reptile neither exposes the developing human fetus to an unwholesome environment nor does it deprive the fetus of a feature that it will later need. Meanwhile the protein that would have been used to develop the scales is used to develop something else in the immature creature.

The beginning to all this takes place in the fallopian tube of the mother as the egg begins it's descent from it's ovary. Several million sperm, inserted by the male, all compete to reach the egg first. Some are deformed and swim in circles, some go the wrong way, others die or fall behind. Only one sperm is destined to reach the egg. If a person is ever called a "loser", there is consolation that we are all winners of a several million sperm race! We are all winners! In a womb, where the supply of protein is not limited, the developmental period is shortened, thus becoming more efficient. Such mutations which are probably due to errors in chromosome replication, or very occasionally may be due to a spontaneous change of carbon-fourteen to nitrogen-fourteen, are very prevalent and usually fatal.

Almost recapitulation
One can imagine that such a process operating over the six hundred million years since the establishment of cellular species would by this time have chipped the fetal process down to the bare essentials. This, indeed, is close to the reality of the case, and some of the recapitulations have been chiseled down to barely recognizable shapes. All multicellular animals have similarities in their first steps of development. The later stages of development contain changes similar to more closely related species. The more closely related the species, the longer the embryonic developments remain similar. Little wonder that some serious students of embryology have gone so far as to deny recapitulation altogether.

Body segments
Often, the more ancient the creature, the more segments it had in its body. The annelid worm had many segments. In the case of the millipede there are over a hundred segments, each with two limbs. Man has been reduced to thirty three segments with only four limbs. The human tail
Each developing human develops a tail, but usually, later reabsorbs it before birth. The stage with the tail represents a monkey stage in our development just before we go into the ape stage. There is a definite division in the development of an individual marked by a birth into prenatal and postnatal periods. The postnatal development progresses right up into the adult, mature stage. This postnatal development is more involved and prolonged in humans, longer than in than any other animal on Earth.

From the beginning
A brief review of part of what is known of embryology may clarify the ontogeny of the developing human fetus. We have some pictures of the first weeks of development. A, B, and C in the illustration represent the fertilization of the human ova. We see the single sperm insert its head into the ovum with its tail sticking out. There are instances in the history of evolution where this tail was retained for purposes of locomotion, but in the case of the human, a negative mutation caused it to dissolve. This did not hurt because the human ovum never has to move around to find nourishment. In figure C we see both the nucleus of the ovum and the sperm before they meld together forming one nucleus.

Fertilization takes place in the fallopian tube of the human mother and it takes two days for the fertilized ovum to pass down into the uterus. In the mean time the ovum divided first into two, then four cells, and more until it becomes a mass of cells rolling along. Since there is no source of nourishment, each cell becomes smaller so that the total mass remains the same.

The solid cluster of cells is called the morula, and corresponds to one of the earliest multicelled colonies in the history of life (F). As we mentioned in the previous chapter, this development allowed for the specialization of cells to perform specific functions. Probably an accident in the genetics of the cells prevented them from separating into individuals, and the cluster had an evolutionary advantage over the single cells. This advantage is reviewed in the embryology of the human.

On the second day, at the time of the hollow ball or blastula, the developing fetus is implanted into the wall of the uterus. The migration of the cells to the periphery of the ball gives each cell an equal opportunity for nourishment. This stage shows first evidence of enlargement. This it can do, for now it is being fed by the mother.

First a dimple and then a pore forms in one side of the blastula. A sac begins to form outside the pore. This is the first of the alantoic sac or bag of waters. This stage is called the gastrula (H), and is reminiscent of a hydra; but, the tentacles of the hydra were lost through a destructive mutation and thus the development of the fetus was simplified. The cells inside the gastrula form the endoderm.

As the alantoic sac forms, it spreads and envelopes the gastrula. Where the alantoic sac touches the gastrula, cells extend from the pore. These are rudimentary skin cells or ectoderm. Just as the hydra had to have sensitive nerve cells around its gastrum, the gastrula develops sensitive cells around its gastrum. These are the rudimentary nerve cells which develop to form the human nervous system. Just underneath the nerve cells forms a layer of cells called the mesoderm.

From the mesoderm forms a cartilaginous rod called the notochord. This is rudimentary of the skeleton and remains with us all our lives as part of the intervertebral discs. It is also reminiscent of the chordata, of which amphiox is an example. Amphiox is a very simple and ancient creature found in the sea.

The three germinal layers
We now have the ectoderm, mesoderm, and endoderm: the three germinal layers from which all the organs of the body form. From the ectoderm form the nervous system, the hair, nails, the skin, and the cornea of the eye. From the mesoderm form the heart, blood vessels, muscles, bones, kidneys, and reproductive organs. From the endoderm branches form the lungs, liver, pancreas, and intestines.

At six weeks the embryos of pigs, sheep, apes, and humans all are very similar in appearance, bespeaking of the long road of evolutionary development shared together. Some nonfunctioning organs remain as vestiges, the appendix, for example. Most of the others do not.

The appendix
The appendix undoubtedly harkens back to the time when we lived in the trees eating leaves as monkeys do. The appendix contained saprophytic organisms which helped in the digestion of cellulose by fermentation, just as they occur in monkeys and rabbits today. The fact that the appendix has not yet disappeared is probably due to the possibility that the genetic code of the appendix is so closely entwined with the vital caecum. A mutation has not been able to separate them without eliminating both, and killing the fetus.

Evolutionary streamlining
Mutations that deform and kill the fetus are very prevalent. About fifty percent of all human pregnancies end in a spontaneous abortions due to genetic and congenital accidents. As much as eleven percent of all children born have malformations. It may be only rarely that a genetic change knocks off an unnecessary developmental process and the fetus survives, but rare occurrences over a long term are effective. We may be sure that this process is active in all creatures of the Earth today. Very probably, this process is most prevalent in humans because their rise in the evolutionary ranks has been most rapid.

We will skip over some of the forms to the five week old specimen, shown above, when it is just short of one centimeter in length. Here we see a creature with a mouth and five folds in the skin in the neck. If we make a cross section of the area of the mouth and pharynx we get a picture as the illustration below it. Here we see five so called brachial arches, each containing an artery. The arteries once carried oxygen to the rest of the body from the gills. With each arch is a pouch called a brachial pouch. This is reminiscent of a shark. Sharks have five brachial clefts, each with gills and filaments on the gills.

In the case of the human embryo, there is evidence of two negative mutations: One that prevented the opening of the pouches into clefts, and another that prevented the formation of filaments and gills, both of which would not serve any purpose in the human. Once in a great while we have a baby born with a "fistula" in its neck where there has been a genetic accident and a partial gill cleft has remained. More often a cyst is all that remains in the neck and this can get infected to the discontentment of the patient.

A shark is a cartilaginous fish, and the fetus is a cartilaginous creature at this time. In fact, all that there is of the skeleton is the simple notochord. There is a feature called heterochronia in embryology which is a condition where evolutionary levels become confused and the embryo still has the skeleton of a chordate and the gill cavities of a much later creature such as a shark. In the special environment of the uterus this can happen without killing the fetus. This is an extreme example of negative mutations chiseling the fetus down to the bare essentials. The lives of millions of fetuses were sacrificed by Nature before this efficient course remained.

Bones and cartilage
It was to be our fate that we evolved from bony fish which swam up rivers to live. Cartilaginous skeletons were not strong enough to withstand the buffeting by the currents of the rivers. That is why river fish have bones rather than cartilage. Most all fish have four ventral fins which became limbs when they crawled up onto land. Something like a lung fish was apparently a step in our line of progress.

It necessarily had to have lungs as well as gills, in this instance, to survive spells when their rivers dried up. They lived in an area of the earth where there were rainy periods followed by long dry spells. During the long dry spells they would encyst in a mud ball with their mouth breathing air through a small hole. They were able to survive until the next rainy spell. During a rain they could creep up onto the land and prey on land insects, retreating into the water when the rain stopped. As time went on, they were able to remain on land longer and longer. Also their ventral fins were modified into feet.

Eventually one type of creature went to dry land permanently and negative mutations wiped out their gills to no disadvantage. These were the first amphibians to invade the land. The human fetus goes through the same stages in a very crude foreshortened way.

From out of the water
The limb buds of the fetus first project laterally (to the sides) like a reptile, and later project ventrally (underneath, to the front) like a mammal. We observe the newborn baby as its nervous system recapitulates these reptilian and mammalian forms. Its limbs will first project laterally (out to the sides) and then ventrally (on hands and knees). The fetus retains a tail right up to the end when, just before partition, it is finally curled under the skin above the anus. Thus, the phases of the mammal, primate, and ape are passed through rather quickly.

Following Birth What better proof of the fact of evolution could Nature leave us than the evolution through many reminiscent steps in our intrauterine development? There is no denigration or irreverence in our knowing this. When Nature created man, this is how it was done. The unstable genetic code is probably built-in in highly complex animals such as man; something like: The higher the steeple, the easier it is to topple. As distressing as this may seem, it also offers much hope in that there is promise of quick progress in just a few hundred years or so with a little selective breeding.

Nature's plan
It is also evident that nature has no great agenda that evolution is following. Developmental trends follow only existing possibilities at the time of their inception. Nature is assiduous in its terminating the lines of the unfit. A creature unable to put together the means for its survival most assuredly becomes extinct. When farmers destroyed the prairie dogs to make way for grain fields, the black-footed ferret, which live on prairie dogs exclusively, also almost disappeared, and may yet go all the way out. The law of evolution is that failures do not survive.