History of Embryology

             Aristotle , in the 4th century  B.C., promoted the idea that the embryo develops from a formless mass that resulted from the union of semen with menstrual blood. Aristotle believed that the male was the major factor in reproduction, although he did grant that the female supplied the matter for shaping. Galen , 2nd  century A.D., wrote a book entitled “ On the formation of the foetus”, also held the two-seed doctrine, claiming both the male and female seeds had coagulative power and receptive capacity for coagulation but that one was stronger in the male and the other in the female.From the time of Galen , until the 16th  century, the prevailed wrong misconception that the embryo will grow in the uterus containing clotted menstrual blood as a seed  till complete growth, with no record of  major advances in the field of embryology.

              In 1651 , William Harvey , One of Fabricius' students at Padua , published his observations , using simple lenses , on the development of chick embryos, and description of the uteri of deer at various stages during mating and pregnancy. He believed that the first evidence of conception appeared  long after the disappearance of semen in the uterus . He concluded that the embryos of the fallow dear  were secreted by the uterus , as he could not see its early stages of development. Harvey postulated, following Aristotle, that the specialised structures of the individual develop step by step from unspecialised antecedents in the egg. This was known as the theory of Epigenesis. Harvey put forward a new theory that  summed up his research, ‘ex ovo omnia’ (‘everything comes from the egg’), implying in effect that the role of parents in generation was indirect.They produced a fertile egg or conceptus or seed, and this subsequently produced a new animal or plant through innate vegetative powers .

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      Other researchers came forth with supporting conclusions, particularly Marcello Malpighi,  the Italian biologist who is considerd the father of modern embryology , published in 1672 his drwings of the developing chick . He thought that the unincubated egg contained a tiny chick and that , in older embryos , new structures gradually make their appearance untill a chiken-like creature is produced .To him , these structures were there all the time;  he was just unable to see them . At about the same time,  in 1672, Regnier de Graaf described little chambers in the uterus of the rabbit and concluded that they must have come from the organs that he called the ovaries. Undoubtedly , these little chambers were what we now call blastocysts.  He observed the changes taking place in rabbits' ovaries in the first days after fertilization and described vesicular follicles in the ovary , which are still called De Graaf’s follicles in his honor. However, he wrongly took the follicle to be the egg. He concluded that similar changes probably took place not only in the rabbit doe but in the human female.

        In 1677, Anton van Leeuwenhoek , who made observations with a simple, early microscope ,  he became interested in male semen after Johan Hamm, a medical student, consulted him about microscopic creatures that he had observed in the semen of a patient suffering from nocturnal emissions. Ham, the real discoverer of sperm,  thought that these  microscopic creatures were evidence of disease .  Leeuwenhoek observed these microscopic creatures, and announced the discovery of  spermatozoa and concluded that they are the larvae of the humans and called them the spermatozoa. Almost immediately after the results of his discoveries were published in the proceedings of the British Royal Society in 1678, all kinds of findings began to be reported.

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        Niklass Hartsoeker's drawing of a human spermatozoon in 1694,  proved that the early microscope  was not sufficient to show the detailed composition of the spermatozoon, and scientists had to complete the picture from their own imagination. They expressed the generally held concept that a fully formed human being in miniature form, or a homunculus, is found in the spermatozoon that was thought to enlarge after the sperm entered the uterus. At that time, scientists held the view that human development was no more than an increase in the size of a single form which enlarged with the development of pregnancy. This belief was due to the domination of the preformation theory. As opposed to the supporters of epigenesis--who hypothesized that life arose from formless matter--the preformationists reasoned that all life, especially human life, was "preformed" at the moment of Creation; that successive generations of individuals were encased, one inside the other, in increasingly small and perfectly formed versions of their adult selves.
   

     In 1875 , ‘Oscar Hertwig concluded from a study of the reproduction of the sea urchin that fertilisation in both animals and plants consists of the physical union of the two nuclei contributed by the male and female parents.’ In 1877 , ‘Hermann Fol , a Swiss zoologist and physician , observed the penetration of the spermatozoan of a starfish into the egg. He was able to see the transfer of the intact nucleus of the sperm into the egg, where it became the male pronucleus.’ In 1878 , Flemming observed the chromosomes and suggested their role in fertilization.In 1883 , von Beneden noticed that the mature germ cells have a reduced number of chromosomes .

    Toward the end of the 19th century the German anatomist Wilhelm Roux, the father of experimental embryology,  as he was the first one to manibulate embryos and observe the effects of these manibulations on them . In 1888 ,   by an experiment conducted on frog eggs , Roux  reported that  the fertilized egg receives substances that represent different characteristics of the organism, which,  as cell division occurs,  become linearly aligned on the chromosomes and are subsequently distributed unequally to daughter cells. This "qualitative division" fixes the fate of the cells and their descendants because some of the determinants are lost to a cell at each division.

        Another German embryologist, Hans Driesch in 1892 , studied the  sea urchin embryos  and found that isolated cells at the    four-cell stage also develop normally and he  concluded that each cell retains all the developmental potential of the zygote. The conflict between these two opposing views of development has been settled in favor of Driesch's interpretation by numerous cell separation experiments and equal distribution of hereditary information to all cells had been established in the end the 19th  century , however ,  its role in development remained an enigma .

       In 1924 the German experimental embryologist Hans Spemann  and his student Hilde Mangold discovered “embryonic induction.” He found that the mesoderm of an undifferentiated embryo was the “organizer” of brain and spinal cord formation from the overlying ectoderm. The essential nature of the organizer  and the process of embryonic induction remained a mystery until recent years. The current progress in understanding embryonic induction has come from research on a class of protein molecules , called growth factors, which were discovered by Rita Levi­Montalcini and Viktor Hamburger in the 1950s. A large number of growth factors are discovered that play important roles in a variety of developmental processes, including embryonic induction.

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