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1 - morula, 2 - blastula
1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm.
Dissection of human embryo, 38 mm - 8 weeks

Embryology (from Greek ἔμβρυον, embryon, "the unborn, embryo"; and -λογία, -logia) is the science of the development of an embryo from the fertilization of the ovum to the fetus stage.

Embryonic development of animals[edit]

After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.


In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus). If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.

The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:

Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.


Humans are bilaterans and deuterostomes.

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

History of embryology[edit]

Human embryo at six weeks gestational age
Histological film 10 day mouse embryo
Beetle larvae

As recently as the 18th century, the prevailing notion in human embryology was preformation: the idea that semen contains an embryo — a preformed, miniature infant, or "homunculus" — that simply becomes larger during development. The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. Much early embryology came from the work of the great Italian anatomists: Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, Mauro Rusconi, etc.[1] According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favoured explanation among embryologists.[2]

After 1827[edit]

8-9 weeks human embryo

Karl Ernst von Baer and Heinz Christian Pander proposed the germ layer theory of development; von Baer discovered the mammalian ovum in 1827.[3][4][5] Modern embryological pioneers include Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham. Other important contributors include William Harvey, Kaspar Friedrich Wolff, Heinz Christian Pander, August Weismann, Gavin de Beer, Ernest Everett Just, and Edward B. Lewis.

After 1950[edit]


Vertebrate and invertebrate embryology[edit]

Many principles of embryology apply to invertebrates as well as to vertebrates.[6] Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage.

Modern embryology research[edit]

Currently, embryology has become an important research area for studying the genetic control of the development process (e.g. morphogens), its link to cell signalling, its importance for the study of certain diseases and mutations and in links to stem cell research.

See also[edit]


  1. ^ Massimo De Felici, Gregorio Siracus, The rise of embryology in Italy: from the Renaissance to the early 20th Century, Int. J. Dev. Biol. 44: 515-521 (2000).
  2. ^ Campbell et al. (p. 987)
  3. ^ K. J. Betteridge (1981). "An historical look at embryo transfer". Reproduction. the Journal of the Society for Reproduction and Fertility 62 (1): 1–13. doi:10.1530/jrf.0.0620001. "Three years later, the Estonian, Karl Ernst von Baer, finally found the true mammalian egg in a pet dog (von Baer, 1827)." 
  4. ^ Lois N. Magner (2005). History of the Life Sciences. New York. Basel: Marcel Dekker. p. 166. ISBN 9780824743604. 
  5. ^ Alex Lopata (2009). "History of the Egg in Embryology". Journal of Mammalian Ova Research 26: 2–9. doi:10.1274/jmor.26.2. 
  6. ^ Parker, Sybil. "Invertebrate Embryology," McGraw-Hill Encyclopedia of Science & Technology (McGraw-Hill 1997).

Embryology - History of embryology as a science." Science Encyclopedia. Web. 06 Nov. 2009. <http://science.jrank.org/pages/2452/Embryology.html>.
"Germ layer." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 06 Nov. 2009 <http://www.britannica.com/EBchecked/topic/230597/germ-layer>.

Further reading[edit]

External links[edit]