(International Journal of Sociology and Social Policy 1999, 19:3/4:22-47)

When do human beings (normally) begin?
"scientific" myths and scientific facts

Dianne N. Irving, M.A., Ph.D.
(copyright February 1999)
Reproduced with Permission

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I. Introduction

The question as to when the physical material dimension of a human being begins via sexual reproduction is strictly a scientific question, and fundamentally should be answered by human embryologists--not by philosophers, bioethicists, theologians, politicians, x-ray technicians, movie stars, or obstetricians and gynecologists. The question as to when a human person begins is a philosophical question. Current discussions on abortion, human embryo research (including cloning, stem cell research, and the formation of mixed-species chimeras), and the use of abortifacients involve specific claims as to when the life of every human being begins. If the "science" used to ground these various discussions is incorrect, then any conclusions will be rendered groundless and invalid. The purpose of this article is to focus primarily on a sampling of the "scientific" myths, and on the objective scientific facts that ought to ground these discussions. At least it will clarify what the actual international consensus of human embryologists is with regard to this relatively simple scientific question. In the final section, I will also address some "scientific" myths that have caused much confusion within the philosophical discussions on "personhood."

II. When does a human being begin?

Getting a handle on just a few basic human embryological terms accurately can considerably clarify the drastic difference between the "scientific" myths that are currently circulating, and the actual objective scientific facts. This would include such basic terms as: "gametogenesis," "oogenesis," "spermatogenesis," "fertilization," "zygote," "embryo," and "blastocyst." Only brief scientific descriptions will be given here for these terms. Further, more complicated, details can be obtained by investigating any well-established human embryology textbook in the library, such as some of those referenced below. Please note that the scientific facts presented here are not simply a matter of my own opinion. They are direct quotes and references from some of the most highly respected human embryology textbooks, and represent a consensus of human embryologists internationally.

A. Basic human embryological facts

To begin with, scientifically something very radical occurs between the processes of gametogenesis and fertilization--the change from a simple part of one human being (i.e., a sperm) and a simple part of another human being (i.e., an oocyte--usually referred to as an "ovum" or "egg"), which simply possess "human life", to a new, genetically unique, newly existing, individual, whole living human being (a single-cell embryonic human zygote). That is, upon fertilization, parts of human beings have actually been transformed into something very different from what they were before; they have been changed into a single, whole humanbeing. During the process of fertilization, the sperm and the oocyte cease to exist as such, and a new human being is produced.

To understand this, it should be remembered that each kind of living organism has a specific number and quality of chromosomes that are characteristic for each member of a species. (The number can vary only slightly if the organism is to survive.) For example, the characteristic number of chromosomes for a member of the human species is 46 (plus or minus, e.g., in human beings with Down's or Turner's syndromes). Every somatic (or, body) cell in a human being has this characteristic number of chromosomes. Even the early germ cells contain 46 chromosomes; it is only their mature forms - the sex gametes, or sperms and oocytes - which will later contain only 23 chromosomes each.1  Sperms and oocytes are derived from primitive germ cells in the developing fetus by means of the process known as "gametogenesis." Because each germ cell normally has 46 chromosomes, the process of "fertilization" can not take place until the total number of chromosomes in each germ cell are cut in half. This is necessary so that after their fusion at fertilization the characteristic number of chromosomes in a single individual member of the human species (46) can be maintained--otherwise we would end up with a monster of some sort.

To accurately see why a sperm or an oocyte are considered as only possessing human life, and not as living human beings themselves, one needs to look at the basic scientific facts involved in the processes of gametogenesis and of fertilization. It may help to keep in mind that the products of gametogenesis and fertilization are very different. The products of gametogenesis are mature sex gametes with only 23 instead of 46 chromosomes. The product of fertilization is a living human being with 46 chromosomes. Gametogenesis refers to the maturation of germ cells, resulting in gametes. Fertilization refers to the initiation of a new human being.

1) Gametogenesis

As the human embryologist Larsen2  states it, gametogenesis is the process that converts primordial germ cells (primitive sex cells) into mature sex gametes--in the male (spermatozoa, or sperms), and in the female (definitive oocytes). The timing of gametogenesis is different in males and in females. The later stages of spermatogenesis in males occur at puberty, and continue throughout adult life. The process involves the production of spermatogonia from the primitive germ cells, which in turn become primary spermatocytes, and finally spermatids--or mature spermatozoa (sperms). These mature sperms will have only half of the number of their original chromosomes--i.e., the number of chromosomes has been cut from 46 to 23, and therefore they are ready to take part in fertilization.3  

Oogenesis begins in the female during fetal life. The total number of primary oocytes--about 7 million--is produced in the female fetus" ovaries by 5 months of gestation in the mother's uterus. By birth, only about 700,000 - 2 million remain. By puberty, only about 400,000 remain. The process includes several stages of maturation--the production of oogonia from primitive germ cells, which in turn become primary oocytes, which become definitive oocytes only at puberty. This definitive oocyte is what is released each month during the female's menstrual period, but it still has 46 chromosomes. In fact, it does not reduce its number of chromosomes until and unless it is fertilized by the sperm, during which process the definitive oocyte becomes a secondary oocyte with only 23 chromosomes.4  

This halving of the number of chromosomes in the oocytes takes place by the process known as meiosis. Many people confuse meiosis with a different process known as mitosis, but there is an important difference. Mitosis refers to the normal division of a somatic or of a germ cell in order to increase the number of those cells during growth and development. The resulting cells contain the same number of chromosomes as the previous cells--in human beings, 46. Meiosis refers to the halving of the number of chromosomes that are normally present in a germ cell - the precursor of a sperm or a definitive oocyte - in order for fertilization to take place. The resulting gamete cells have only half of the number of chromosomes as the previous cells--in human beings, 23.

One of the best and most technically accurate explanations for this critical process of gametogenesis is by Ronan O'Rahilly,5  the human embryologist who helped to develop the classic Carnegie stages of human embryological development. He also sits on the international board of Nomina Embryologica (which determines the correct terminology to be used in human embryology textbooks internationally):

"Gametogenesis is the production of [gametes], i.e., spermatozoa and oocytes. These cells are produced in the gonads, i.e., the testes and ovaries respectively. … During the differentiation of gametes, diploid cells (those with a double set of chromosomes, as found in somatic cells [46 chromosomes]) are termed primary, and haploid cells (those with a single set of chromosomes [23 chromosomes]) are called secondary. The reduction of chromosomal number … from 46 (the diploid number or 2n) to 23 (the haploid number or n) is accomplished by a cellular division termed meiosis. … Spermatogenesis, the production of spermatozoa, continues from immediately after puberty until old age. It takes place in the testis, which is also an endocrine gland, the interstitial cells of which secrete testosterone. Previous to puberty, spermatogonia in the simiferous tubules of the testis remain relatively inactive. After puberty, under stimulation from the interstitial cells, spermatogonia proliferate … and some become primary spermatocytes. When these undergo their first maturation division (meiosis 1), they become secondary spermatocytes. The second maturation division (meiosis 2) results in spermatids, which become converted into spermatozoa."6  

"Oogenesis is the production and maturation of oocytes, i.e.; the female gametes derived from oogonia. Oogonia (derived from primordial germ cells) multiply by mitosis and become primary oocytes. The number of oogonia increases to nearly seven million by the middle of prenatal life, after which it diminishes to about two million at birth. From these, several thousand oocytes are derived, several hundred of which mature and are liberated (ovulated) during a reproductive period of some thirty years. Prophase of meiosis 1 begins during fetal life but ceases at the diplotene state, which persists during childhood. … After puberty, meiosis 1 is resumed and a secondary oocyte … is formed, together with polar body 1, which can be regarded as an oocyte having a reduced share of cytoplasm. The secondary oocyte is a female gamete in which the first meiotic division is completed and the second has begun. From oogonium to secondary oocyte takes from about 12 to 50 years to be completed. Meiosis 2 is terminated after rupture of the follicle (ovulation) but only if a spermatozoon penetrates. … The term "ovum" implies that polar body 2 has been given off, which event is usually delayed until the oocyte has been penetrated by a spermatozoon (i.e., has been fertilized). Hence a human ovum does not [really] exist. Moreover the term has been used for such disparate structures as an oocyte and a three-week embryo, and therefore should be discarded, as a fortiori should "egg"."7  (Emphasis added.)

Thus, for fertilization to be accomplished, a mature sperm and a mature human oocyte are needed. Before fertilization,8  each has only 23 chromosomes. They each possess "human life," since they are parts of a living human being; but they are not each whole living human beings themselves. They each have only 23 chromosomes, not 46 chromosomes--the number of chromosomes necessary and characteristic for a single individual member of the human species. Furthermore, a sperm can produce only "sperm" proteins and enzymes; an oocyte can produce only "oocyte" proteins and enzymes; neither alone is or can produce a human being with 46 chromosomes.

Also, note O'Rahilly's statement that the use of terms such as "ovum" and "egg"--which would include the term "fertilized egg"--is scientifically incorrect, has no objective correlate in reality, and is therefore very misleading--especially in these present discussions. Thus these terms themselves would qualify as "scientific" myths. The commonly used term, "fertilized egg," is especially very misleading, since there is really no longer an egg (or oocyte) once fertilization has begun. What is being called a "fertilized egg" is not an egg of any sort; it is a human being.

2) Fertilization

Now that we have looked at the formation of the mature haploid sex gametes, the next important process to consider is fertilization. O'Rahilly defines fertilization as:

"… the procession of events that begins when a spermatozoon makes contact with a secondary oocyte or its investments, and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote. The zygote is characteristic of the last phase of fertilization and is identified by the first cleavage spindle. It is a unicellular embryo."9  (Emphasis added.)

The fusion of the sperm (with 23 chromosomes) and the oocyte (with 23 chromosomes) at fertilization results in a live human being, a single-cell human zygote, with 46 chromosomes--the number of chromosomes characteristic of an individual member of the human species. Quoting Moore:

"Zygote: This cell results from the union of an oocyte and a sperm. A zygote is the beginning of a new human being (i.e., an embryo). The expression fertilized ovum refers to a secondary oocyte that is impregnated by a sperm; when fertilization is complete, the oocyte becomes a zygote."10  (Emphasis added.)

This new single-cell human being immediately produces specifically human proteins and enzymes11  (not carrot or frog enzymes and proteins), and genetically directs his/her own growth and development. (In fact, this genetic growth and development has been proven not to be directed by the mother.)12  Finally, this new human being--the single-cell human zygote--is biologically an individual, a living organism--an individual member of the human species. Quoting Larsen:

"… [W]e begin our description of the developing human with the formation and differentiation of the male and female sex cells or gametes, which will unite at fertilization to initiate the embryonic development of a new individual."13  (Emphasis added.)

In sum, a mature human sperm and a mature human oocyte are products of gametogenesis--each has only 23 chromosomes. They each have only half of the required number of chromosomes for a human being. They cannot singly develop further into human beings. They produce only "gamete" proteins and enzymes. They do not direct their own growth and development. And they are not individuals, i.e., members of the human species. They are only parts--each one a part of a human being. On the other hand, a human being is the immediate product of fertilization. As such he/she is a single-cell embryonic zygote, an organism with 46 chromosomes, the number required of a member of the human species. This human being immediately produces specifically human proteins and enzymes, directs his/her own further growth and development as human, and is a new, genetically unique, newly existing, live human individual.

After fertilization the single-cell human embryo doesn't become another kind of thing. It simply divides and grows bigger and bigger, developing through several stages as an embryo over an 8-week period. Several of these developmental stages of the growing embryo are given special names, e.g., a morula (about 4 days), a blastocyst (5-7 days), a bilaminar (two layer) embryo (during the second week), and a trilaminar (3-layer) embryo (during the third week).14  

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nov 17/Jun/00