Perhaps the first paper written on the topic of embryology was by Aristotle (384-322 B.C.). He observed the development of chicken eggs very closely and based his theories of embryonic growth on these observations. He theorized that embryonic development could occur in two ways: preformation or epigenesis.
The Preformation Theory
. This theory postulated that each female egg contained a miniature human being (homunculus). According to this theory, all that would have to happen was for the male’s seed to unite with the female’s egg to start the development of this miniature person. For a long period of time many prominent people subscribed to this theory and some went as far as to figure out how many preformed humans were carried by Eve (200 million). They claimed that after all of these preformed people were developed, our race would end.
The Epigenesis Theory
. According to this theory Aristotle believed that the young organism developed from a formless mass of living material. He postulated that development of this mass into a human being would occur by differentiation of cells into various body parts. We see a degree of proof of this differentiation by studying the three layers of zygote (ectoderm, mesoderm, and endoderm).
In the late nineteenth century two men, Wilhelm Roux (1850-1924) and Hans Driesch (1867-1941) formulated their own theories of embryonic development. Roux’s experiments were designed to prove that a modified theory of preformation was correct. In his Mosaic Theory of embryonic development he claimed that if there is a preformed pattern within the fertilized egg, then one cell remaining alive would form only one part of an embryo. Roux went on to kill one cell and only developed half embryos out of the remaining cells.
Meanwhile, Driesch set out to disprove the Mosaic Theory of preformation and put forth his own theory of preformation. His experiments were done on sea urchins, unlike Roux he did not kill any cells, he merely removed one cell and developed it. Both of the remaining cells developed into complete sea urchins, as a matter of fact in sea urchins it has been discovered that this is possible even up to where the zygote contains four cells. If further experiments take place after the four cell development stage, just as in Roux’s results, you’d get incomplete embryos. Driesch was fortunate in his selection of experimental animals because such cell division is limited to just a few species.
Modern scientists have found both men to be partially correct with their theories—correct in that the embryo is “preformed”, however, this occurs in the form of the inherited genetic code of deoxyribonucleric acid (DNA). Research has proven that the “blueprint” for development is contained in the DNA.
Interesting Facts on Human Embryology
. One can not but be amazed by the following facts surrounding prenatal development: (1) the original weight of the human zygote increases more than a billion times during the 266 days from conception to birth (imagine if the earth suddenly began to grow at the same rate; in less than a year it would be approximately 20 million times the weight of the sun); (2) from the original two simple germ cells, the egg and sperm, comes a nature human being consisting of 30 billion differentiated cells performing very complex tasks; (3) locked within those two germ cells are genetic codes that represent some traits of all of our ancestors; (4) from these two cells can come another Einstein, Bach, Rembrant, Curie, or Marian Anderson. Man has long had a desire to understand human conception and embryology, and the more he delves, the more he comprehends how much there is to learn. Two interesting unknowns are: (1) scientists are unsure of exactly how the intrauterine device (IUD) works, what mechanism does it trigger so as not to allow the zygote to implant itself in the endometrium,
and (2) there is little or no knowledge of how the egg is capable of allowing only one sperm to enter it when in fact it is being bombarded by millions of them.
These and the many other facts and topics in this unit can be very enthralling to children of all ages and especially to those approaching reproductive age.
It goes without saying that a great deal of knowledge and data has been accumulated in this field, but like any other investigative science, many frontiers are still ahead.