Human Development

What Embryology and Science textbooks say about the Beginning of Human Beings

"Just the Facts, Ma'am": It's too bad Sergeant Friday isn't around to help us with the embryonic stem cell debate.

by Clark Forsythe, AUL

Much of the public promotion for human embryonic research rests on the assumption that the human embryo is not a human being.

As a May 2004 letter to USA Today put it (inaccurately), "a gamete about the size of a period at the end of a sentence" is not a human being. [True, a gamete (sex cell, egg or sperm) is not a human being. But the target of embryo research is not gametes; it's embryos.]

Here's a reveiw of what contemporary embryology and biology texts say about the beginning of human beings.

William Larsen in his 1993 text, Human Embryology, states: "the nuclei of the male and female gametes unite, resulting in the formation of a zygote containing a single diploid [having the full complement of chromosomes] nucleus. Embryonic development is considered to begin at this point." Larsen also states that " the moment of zygote formation may be taken as the beginning or zero time point of embryonic development."

Bruce Carlson in his 1994 text, Human Embryology and Developmental Biology, states that "through the mingling of maternal and paternal chromosomes, the zygote is a genetically unique product of chromosomal reassortment." Carlson states that when "the maternal and paternal chromosomes…become organized around the mitotic spindle in preparation for an ordinary mitotic division…the process of fertilization can be said to be complete and the fertilized egg is called a zygote."

The one-celled human zygote is properly called an embryo.

What is the nature of this embryo?

O'Rahilly & Muller in their 1994 text, Human Embryology & Teratology, state: "the zygote…is a unicellular embryo and a highly specialized cell." They explain that "[a]lthough life is a continous process, fertilization is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is thereby formed."

They add: "this remains true even though the embryonic genome is not actually activated until 4-8 cells are present, at about 2-3 days." Furthermore, "[t]he embryonic genome is formed" with the union of the male and female pronuclei, and, at that point, "the embryo now exists as a genetic unity."

So, this one-celled embryo is a "genetically distinct human organism" which "exists as a genetic unity". O'Rahilly & Muller quote from a pioneering work in human embryology by Heuser & Streeter: "[I]t is to be remembered that at all stages the embryo is a living organism, that is, it is a going concern with adequate mechanisms for its maintenance as of that time."

The human zygote or embryo has undoubted genetic individuality.

The developing embryo's sex and its separate and individual genetic identity are determined at fertilization.

As Pauerstein states in his 1987 text on Obstetrics, "[e]ach member of a species begins with fertilization — the successful merging of two different pools of genetic information to form a new individual."

Levine & Suzuki concur in their 1993 text, The Secret of Life: "For better or for worse, every individual's genetic endowment is determined at the moment of conception. Sperm and egg each carry a random selection of parental genes. Their fusion creates a genetically unique individual."

Another name for organism is being. It is a human being at the embryonic stage.

As the 1995 Ramsey Colloquium on Embryo Research concluded: "The embryo is a being: that is to say, it is an integral whole with actual existence. The being is human. It will not articulate itself into some other kind of animal. If is is objected that, at five or fifteen days, the embryo does not look like a human being, it must be pointed out that this is precisely what a human being looks like — and what each of us looked like — at five or fifteen days of development."

These are the concepts and language that human embryologists have used in the 20th century. The size of the human organism, and the uses to which it can be put, don't change the scientific reality.

[Clark Forsythe, Dir, AUL Project in Law and Bioethics]