http://www.consciencelaws.org/Examining-Conscience-Background/GenScience/BackGenScience06.html#02
The scientific facts
Before fertilization there exist a human sperm (containing 23 chromosomes) and a human ovum (also containing 23 chromosomes -- the same number, but different kinds of chromosomes).24 Neither the sperm nor the ovum, singly, by itself, can become a human being -- even if implanted in the womb of the mother. They are only gametes -- they are not human embryos or human beings. In contrast, the single-cell embryonic human zygote formed after fertilization (the beginning of the human being and the embryonic period)25 contains 46 chromosomes (the number of chromosomes which is specific for members of the human species) -- and these 46 chromosomes are mixed differently from the 46 chromosomes as found in either the mother or the father -- that is, they are unique for that human individual. And at the single-cell embryonic human zygote stage that unique individual human being is already genetically a girl or a boy.26 If allowed to "do his or her own thing", so to speak, this embryonic human zygote will biologically develop continuously without any biological interruptions, or gaps, throughout the embryonic, fetal, neo-natal, childhood and adulthood stages -- until the death of the organism. And with the advent of in vitro fertilization techniques, we can see that the early human embryo can develop in vitro on his or her own without the nutrition or protection of the mother for quite a while -- someday, perhaps, even until "birth"!
I want to reiterate that a human gamete is not a human being or a human person. The number of chromosomes is only 23; it only acts or functions biologically as an ovum or as a sperm, e.g., it only makes ovum or sperm enzymes and proteins, etc., not specifically human enzymes and proteins; and by itself it does not have the actual nature or potency yet to develop into a human embryo, fetus, child, or adult. And in that sense gametes are only possible human beings (i.e., human beings who do not exist as yet). Only after the sperm and the ovum chromosomes combine properly and completely do we have a human being. Individually, the nature of a sperm is different from the nature of an ovum -- and both are different from the nature of the embryonic human zygote which is formed when their chromosomes combine.
Thus from perhaps an Aristotle-the-biologist's point of view, one would say that before fertilization there are two natures -- i.e., the nature of an ovum and the nature of a sperm. After fertilization there is a human zygote with one nature, i.e., the nature of a human being. Thus, in fertilization there is substantial change,27 (i.e., a change in substance or nature -- or "what" it is). The substances or natures of the ovum and the sperm have changed into the nature of a human being. This is, in fact, known empirically by observing the number and kinds of chromosomes present before and after fertilization, and by empirically observing the different characteristically specific actions and functions of the ovum, the sperm, and the human zygote. Once fertilization has taken place and the new human being has formed, only accidental change28 occurs (e.g., a change in weight, height, size, shape, etc.), and we know this empirically as well. We can observe that the nature of the human being does not change (e.g., into a cabbage or a giraffe), only its human accidents change.
Thus embryological development does not entail substantial change, but only accidental change. Once it is a human being it stays a human being, and acts and functions biologically as a human being. The human zygote produces specifically human enzymes and proteins; he or she forms specifically human tissues and organ systems, and develops humanly continuously from the stage of a single-cell human zygotic embryo to the stage of a human adult.29
This is observed empirically. A human zygote does not produce cabbage or carrot enzymes or proteins, and does not develop into a rock, an ear of corn, nor into a cat, a horse, a chicken, or a giraffe. Empirically it is observed that a human zygote produces specifically and characteristically human proteins and enzymes at the moment of fertilization -- as demonstrated recently, for example, by experiments using transgenic mice30 -- and that he or she develops continuously throughout embryological development in a specifically and characteristically human way.
In short -- the biological facts demonstrate that at fertilization we have a real human being with a truly human nature. It is not that he or she will become a human being -- he or she already is a human being. We know that empirically. And this nature or capacity to act in a certain characteristic way is called, philosophically, a nature or a potency.31 Thus a human zygote or embryo is not a possible human being;32 nor is he or she a potential human being;33 he or she is already a human being. A human zygote, embryo or fetus does not have the potency to become a human being, but already possesses the nature or capacity to be at that moment a human being. And that nature will direct the accidental development, i.e., the embryological development, of his or her own self from the most immature stage of a human being to the most mature stage of a human being.
Now, this is strongly convincing empirical evidence that at fertilization there is present a human being (the well-referenced unequivocally agreed upon answer to the scientific question); but is there also a human person (a philosophical question) -- or not? These are two different questions -- one scientific, the other philosophical. It is in this shifting from the paradigm of a human being to that of a human person where the philosophy -- and the confusion -- come into play. Is a human being also a human person; or are they different things? Which philosophy is adequate to cope with this biological data?
Now, this is strongly convincing empirical evidence that at fertilization there is present a human being (the well-referenced unequivocally agreed upon answer to the scientific question); but is there also a human person (a philosophical question) -- or not? These are two different questions -- one scientific, the other philosophical. It is in this shifting from the paradigm of a human being to that of a human person where the philosophy -- and the confusion -- come into play. Is a human being also a human person; or are they different things? Which philosophy is adequate to cope with this biological data?