Similarities In Embryonic Structures

The branch of science that studies the phases of embryonic development of an organism from the stage of zygote to fetus is called “embryology” or “ontogeny”. The branch of science that studies the changes and developments of the species that an organism belongs to and that are supposed to have taken place beginning from the moment that organism appeared on the earth to the present day during the geological eras is called “phylogeny”.   

Reasoning that community in embryonic structure reveals community of descent, Darwin thought, the early embryos, “show us, more or less completely, the condition of the progenitor of the whole group in its adult state”. Darwin was not an embryologist, however, so he relied on the work of German biologist Ernst Haeckel (1834-1919).

Haeckel produced drawings of embryos from various classes of vertebrates to show that they are virtually identical in their earliest stages, and become noticeably different only as they develop (Figure l). 

Figure 1.  Haeckel’s embryos. A- Fish, B- Salamander, C- Turtle, D- Chick, E- Pig, F- Cow, G- Rabit, H- Human. Haeckel’s drawings above are from George Romanes's 1892 book called “Darwinism Illustrated” (You can see that in all of the books of today, these drawings that were drawn by Haeckel about 140 years ago are used).

Before the publication of “the Origin of Species” by Darwin in 1859, Europe’s most famous embryologist was not Ernst Haeckel but Karl Ernst von Baer (1792-1876). Ernst von Baer, who was trained in physics and biology, published his work, which would have a significant role in the discussions about evolution later, in embryology in 1830s¹.

In this work, von Baer refuted “Prefor­mationism”, the old idea that embryos are simply miniature adults. Regarding the development of the embryo, he put forward the following idea: “The more general characters of a large group of animals appear earlier in their embryos than the more special characters”².

According to the view introduced by von Baer called “the Recapitulation Theory” at first and renamed as “the Biogenetic Law” by E. Haeckel later in 1886, ontogeny recapitulates phylogeny³.

In his book called “General Morphology of Organism” published in 1876, Haeckel says, “Ontogeny summarizes phylogeny”. His claim was that the human embryo started like a single-celled sea organism, then it developed into a worm with a tube shaped heart that pulsated, then into a gilled fish with a two-chambered heart, then into an amphibian with a three-chambered heart, then into a mammal with a four-chambered heart and a tail, and finally into a human baby ⁴.

That is, during its ontogenetic development, an individual undergoes the phases that its genus underwent during its philogeny.  The similarities that vertebrate show during their embryonic development are given as examples. It is claimed that the embryos of fish, salamander, turtle, chick, cow, rabbit and human resemble one another during early stages.

It is claimed that they all have similar segmental somites and gill slits. It is supposed that in the embryonic stages that mammals undergo when their kidneys form, first pronephric, then mesonephric development of the embryo takes place and in the later stages, metanephric kidney structure is seen, and that the same embryonic stages repeat in the formation of the kidneys of all species of mammals.   

The examples of recapitulation in plants are fewer than animals. The development of Acacia tree can be given as an example. In the seedlings of Acacia tree, the first leaves that develop after cotyledons are in the form of simple leaves. The leaves of the tree later produce compound leaves. Thus, Acacia is regarded to have developed from simple-leaved ancestors.

CRITICISM OF SIMILARITIES IN
EMBRYONIC STRUCTURES

Gill Slits

The view put forward by those who believe in embryonic recapitulation that “the embryos of all mammals, birds, reptiles and humans had gill slits at a certain stage” is an exaggerated claim. The structures resembling bars and grooves in the neck region mentioned above are called pharyngeal pouches. Gish states that they develop into gills in fish. In the human, they do not open into the throat. Besides, they do not develop into tissues related to respiration. Therefore, it is stated that they cannot be regarded as gill slits⁵.

These slits develop into the lower jaw, Eustachian tube, thymus and parathyroid glands. Therefore, it is not possible to consider them as “useless, vestigial organs”.

According to Haeckel’s biogenetic law, a human becomes a fish first, an amphibian after that, and in the end, takes the form of a human by climbing the ladder of evolution.

Since 1859, when Darwin published his book “the Origin of Species”, it has been claimed that the slits in the human embryo are “gill slits inherited from fish”. Due to their approach that “the human embryo underwent those stages claimed by Haeckel”, evolutionist biologists interpret the development stages of the human embryo in biology textbooks in a way to confirm that view.

Curtis and Barnes state the following in their book, “Invitation to Biology”:

“Early vertebrate embryos are almost indistinguishable. All have prominent gill pouches”⁶.

Gould and Keeton inform the students as follows in their book called “Biological Science”:

“Traces of their genealogy are obvious in vertebrates. Human embryos, for instance, have gill pouches” ⁷.

Raven and Johnson state the following in their book “Biology”:

“Early in their development, human embryos possess gill slits, like a fish” ⁸.

Futuyma emphasizes the same issue in his book, “Evolutionary Biology”:

“Early in development, human embryos are almost indistinguishable from those of fishes, and briefly display gill slits” ⁹.

2- Pharyngeal Pouches

During the embryonic development, there exists a series of folds in the neck and pharynx region of all vertebrate embryos. The convex parts of the folds are called pharyngeal "arches" or "ridges," and the concave parts are called pharyngeal "clefts" or "pouches". In fish, the pharyngeal folds develop into gills. However, in reptiles, mammals and birds, these folds develop into inner ear or parathyroid gland. 

British embryologist Lewis Wolpert states the following regarding gill slits:  

“There is no embryological reason to call pharyngeal pouches “gill-like” in the embryos of vertebrate. The only justification for that term is the theoretical claim that ‘mammals evolved from fish-like ancestors’” ¹.

Swiss embryologist Günter Rager also states that there are no gill slits in humans:

“The only way to see 'gill-like' structures in human embryos is to read evolution into development" and that "there is no way 'gill-slits' in human embryos can logically serve as evidence for evolution” ¹⁰.

Austrian molecular biologist Denton attracts attention to the fact that there is no relation between the development of the vertebrate kidney and the fish and reptiles by saying:

“In fish and amphibia, the kidney is derived directly from an embryonic organ known as the mesonephros, while in reptiles and mammals the mesonephros degenerates towards the end of embryonic life and plays no role in the formation of the adult kidney, which is formed instead from a discrete spherical mass of mesodermal tissue, the metanephros, which develops quite independently from the mesonephros” ¹¹.

If the claim that “ontogeny recapitulates phylogeny” is assumed to be true, such a change is expected to take place in all structures of the organism. For instance, the human heart should have been one-chambered first, and then two, three and four-chambered. However, its development starts with two chambers… Then, it becomes one-chambered. In the end, it develops into a four-chambered heart. The same development is observed in the formation of the brain, too.  The brain develops before the nerve chord and the heart develops before the blood veins, refuting the claim, “ontogeny recapitulates phylogeny”.

White states the following regarding the developments of the human heart:

“It is true that the human embryo starts with a one-chambered heart and then develops into a two chambered-state. However, it goes back to the one-chambered state again before it develops into the three-chambered and finally the four-chambered state” ¹².

It is a great claim that the difference of leaves in the young and adult plants is presented as evidence for the Recapitulation Theory because it is possible to find leaves of various forms in the same oak tree.

Drawings

The embryonic similarities between organisms put forward by the Recapitulation Theory of Haeckel in 1876 have appeared in almost all of the evolution books published since then¹³.

Historian of science Timothy Lenoir states that although von Baer, who published an important book on this issue before Haeckel, accepted the possibility of limited transformation of species at lower levels of the biological hierarchy, he saw no evidence for the large-scale transformations as proposed by Darwin. Lenoir claims that Darwin distorted Baer’s embryological evidence to make it fit his theory. However, Lenoir notes that Darwin persisted in citing him as “the best supporter of his theory of evolution”; Lenoir also states that von Baer was a strong critic of Darwinian evolution until his death¹⁴.

British zoologist Adam Segwick stated in his book he wrote in 1909 that the laws of von Baer and the biogenetic law of Haeckel were very different. He pointed out that the laws of von Baer refuted the theories that were not based on empirical observation and that did not fit the evidence, and that the Biogenetic Law of Haeckel or the Recapitulation was deduced from evolutionary theory rather than inferred from evidence¹⁵.

American embryologist Lillie also states that the Biogenetic Law of Haeckel is a logical consequence of the Theory of Evolution rather than an empirical inference¹⁶.

Historian of science Rasmussen states that from the very beginning, the Biogenetic Law was based on a theoretical deduction rather than facts and that the reasons of rejection were present beginning from the first days it was put forward, and adds the following:

“All the important evidence called upon in the rejection of the biogenetic law was there from the first days of the law's acceptance” ¹⁷.

British embryologist Gavin de Beer states in his book on embryology and evolution in which he criticized Haeckel’s Biogenetic Law that Recapitulation, that is, the pressing back of adult ancestral stages into early stages of development of descendants, does not take place and that recapitulation is a “mental strait-jacket” that has thwarted and delayed embryological research¹⁸.

Wells points out that both views have continually been on the agenda throughout the twentieth century and adds the following:

“Both views are empirically false. Yet throughout the twentieth century, they have periodically risen, phoenix-like, from the ashes of empirical disconfirmation. Since both are frequently enlisted in support of Darwinian evolution, it is often difficult to tell them apart; they are one of the most bizarre twists of all and both are now illustrated with the same set of faked drawings” ¹.

Oppenheimer says Haeckel altered the embryo drawings so as to support the view of “evolution” and adds:

“Haeckel used the same woodcut to print embryos that were supposedly from different classes. In others, he doctored his drawings to make the embryos appear more alike than they really were. Haeckel’s contemporaries repeatedly criticized him for these misrepresentations and accused him of fraud” ¹⁹.

British embryologist states in his book he published in 1995 that Haeckel misrepresented the truth in his drawings and adds:

“Whether or not Haeckel was guilty of fraud—that is, deliberate deception—there is no doubt that his drawings misrepresent vertebrate embryos. First, he chose only those embryos that came closest to fitting his theory. Although there are seven classes of vertebrates (jawless fishes, cartilaginous fishes, bony fishes, amphibians, reptiles, birds, and mammals), Haeckel showed only five, omitting jawless and cartilaginous fishes entirely. Furthermore, to represent amphibians he used a salamander rather than a frog, which looks very different. Finally, half of his embryos are mammals, and all of these are from one order (placentals); other mammalian orders (egg-laying monotremes and pouch-brooding marsupials) are omitted. Thus, Haeckel began with a biased sample” ²⁰.

Richardson and his colleagues explain the reason why Haeckel chose salamanders in his studies of the embryo as follows:

“There is a great variation in embryonic morphology among amphibians, but Haeckel chose a salamander that happened to fit his theory”²¹.

Richardson and his colleagues also found that vertebrate embryos vary tremendously in size, from less than one millimeter to almost ten millimeters, yet Haeckel portrayed them all as being the same size²².

Famous evolutionist wrote in the March 200, issue of “Natural History” that Haeckel distorted the drawings of the development stages of the vertebrate embryos stating the following:

“Haeckel exaggerated the similarities by idealizations and omissions. His drawings are characterized by inaccuracies and outright falsification”²³.

In an interview made by “Science” magazine, Gould says what Haec­kel made in the drawings of vertebrate embryos was fake:  

“What Haeckel did turned out to be one of the most famous fakes in biology. Briefly, Haeckel’s drawings are fakes, and they misinterpret the embryos they purport to show. Darwin based his inference of common ancestry on the belief that the earliest stages or embryo development are the most similar. Haeckel’s drawings, however, omit the earliest stages entirely, and start at a point midway through the development. The earlier stages are much different” ¹.

As a historian of science, Gould wrote a major book on the subject in 1977, “Ontogeny and Phylogeny”. Instead of criticizing and blaming such frauds, Gould approves them and argues that such scientific frauds are useful because they excite the imagination:

“Tales of scientific fraud excite the imagination for good reason. Getting away with this academic equivalent of murder for generations, and then being outed a century after your misdeeds, makes even better copy” ²³.

Wells asks the following question about Haeckel’s fake drawings:

“If biologists have known all along that Haeckel’s drawings were faked, then why are they still used?”

He answers the question in Gould’s words:

“Gould laid the blame at the feet of textbook writers, blasting them for dumbing down their subject matter to the point of making it inaccurate. Gould added, ‘We should be both astonished and ashamed by the century of mindless recycling that has led to the persistence of these drawings in a large number of modern textbooks!’”²⁴.

Hitching states that Ernst Haeckel himself admitted that the drawings of these embryos did not represent the truth and quotes his following view:

“After this compromising confession of 'forgery' I should be obliged to consider myself condemned and annihilated if I had not the consolation of seeing side by side with me in the prisoner's dock hundreds of fellow-culprits, among them many of the most trusted observers and most esteemed biologists. The great majority of all the diagrams in the best biological textbooks, treatises and journals would incur in the same degree the charge of 'forgery,' for all of them are inexact, and are more or less doctored, schematized and constructed”²⁵.

Famous evolutionist George Simpson also states that Haeckel made mistakes:

“Haeckel presented the development of evolution wrongly. It is definitely known today that the embryonic developments of the living beings do not represent their past” ³⁷.

Keith states that the Recapitulation Theory was excluded from the textbooks in the 1950s:

“Surely the biogenetic law (the Recapitulation Theory) is as dead as a doornail. It was finally exorcised from biology textbooks in the 1950s” ²⁶.

Wells points out that the earliest stages in the vertebrates are not the most similar ones and states the following:

“When an animal egg is fertilized, it first undergoes a process called "cleavage", during which it subdivides into hundreds or thousands of separate cells without growing in overall size. At the end of cleavage, the cells begin to move and rearrange themselves in the stage known as gastrulation. Gastrulation even more than cleavage is responsible for establishing the animal's general body plan (e.g., insect or vertebrate) and for generating basic tissue types and organ systems (e.g., skin, muscles and gut)

If it were true (as Darwin and Haeckel claimed) that vertebrates are most similar in their earliest stages, then the various classes would be most similar during cleavage and gastrulation. Yet a survey of bony fish, amphibian, reptile, bird and mammal reveals that this is not the case.

In the gastrulation stage, a fish is very different from an amphibian, and both are very different from reptiles, birds, and mammals. Whatever model is examined here, it does not comply with the view that ‘the earliest stages are the most similar and later stages are more different’”¹.

Embryologist Ballard states the difference in the development stages of the embryos as follows:

“It is only by semantic tricks and subjective selection of evidence, by bending the facts of nature, that one can argue that the early embryo stages of vertebrates are more alike than their adults”²⁷.

 Embryologist Blechschmidt and Elinson report that frogs, chicks, and mice are radically different in such fundamental properties as egg size, fertilization mechanisms, cleavage patterns, and gastrulation movements^28-29.

Although the development of the embryos does not support the views of Darwin and Haeckel, the embryologic development stages are interpreted through an evolutionist view and this interpretation is present in almost all biology books. 

Well states it as follows:

“Since Darwin’s theory is affirmed regardless of the evidence, and “ontogeny recapitulates phylogeny” is a logical deduction from that theory, biology textbooks continue to teach it - though they usually attach von Baer's name to it. Thus, the 1975 edition of B. I. Balinsky’s classic textbook, Introduction to Embryology, includes this amazing passage:

“‘Von Baer’s law’ can be reinterpreted in the light of evolutionary theory. In its new form, the law is known as the ‘biogenetic law of Haeckel’. According to von Baer’s law, features of ancient origin develop early in ontogeny; features of newer origin develop late. Hence, the ontogenetic development presents the various features of the animal’s organization in the same sequence as they evolve during the phylogenetic development, ontogeny recapitulates phylogeny³⁰.

“It is difficult to imagine how any history of the biogenetic law could be more distorted than this. Yet the distortion is perpetuated in many modern biology textbooks. And as if this were not bad enough, some textbooks even use Haeckel's faked drawings to illustrate von Baer's law. For example, Haeckel's drawings are reproduced in the 1998 edition of Douglas Futuyma's advanced college textbook, Evolutionary Biology, but the figure caption doesn't mention Haeckel; instead, it describes the drawings as ‘an illustration of von Baer’s law’”⁹.

In 2000, this attitude of Futuyma was criticized as “lying by distorting the truth”. In his defense, Futuyma explained that he had been unaware of the discrepancies between Haeckel's drawings and the actual vertebrate embryos"³¹.

Wells states the following regarding the issue:

“So Futuyma, a professional evolutionary biologist and author of a graduate-level textbook, did not know about Haeckel's faked drawings — a confession of ignorance not likely to inspire much confidence in the quality of our biology textbooks. But now he knows that “Haeckel was inaccurate and misleading,” and he said he would take this into account in future editions of his book”³².

In their book, “Invitation to Biology”, Helena Curtis and Sue Barnes reproduced the top two lines of Haeckel’s drawings with the following caption:

“These drawings are based on the work of the nineteenth-century embryologist Karl Ernst von Baer”⁶.

 Yet falsely attributing Haeckel’s ideas and drawings to von Baer is not the most serious offense in these textbooks. That distinction goes to their use of Haeckel’s drawings to misrepresent the embryological evidence 

According to Wells, Haeckel's drawings are misleading in three ways:

1. They include only those classes and orders that come closest to fitting Haeckel's theory.

2. They distort the embryos they purport to show.

3. They entirely omit earlier stages in which vertebrate embryos look different.

Haeckel’ s drawings appear not only in Futuyma's book but also in the latest edition of “Molecular Biology of the Cell”, by National Academy of Sciences President Bruce Alberts and his colleagues. Albert states the following regarding the issue:

“Early developmental stages of animals whose adult forms appear radically different are often surprisingly similar, and that Darwinian evolution explains why embryos of different species so often resemble each other in their early stages and, as they develop, seem sometimes to replay the steps of evolution”³³.

Many textbooks use slightly redrawn versions of Haeckel’s embryos. One example is the 1999 edition of Peter Raven and George Johnson’s Biology, which accompanies its drawings with the following caption:

 “Notice that the early embryonic stages of these vertebrates bear a striking resemblance to each other.”

 The book also informs students:

 “Some of the strongest anatomical evidence supporting evolution comes from comparisons of how organisms develop. In many cases, the evolutionary history of an organism can be seen to unfold during its development with the embryo exhibiting characteristics of the embryos of its ancestors”⁸.

Another example includes the 1998 edition of Cecie Starr and Ralph Taggart’s Biology: the Unity and Diversity of Life, which accompanies its drawings with this misstatement:

 “The early embryos of vertebrates strongly resemble one another”³⁴.

 The latest edition of James Gould and William Keeton’s Biological Science includes this statement:

“One fact of embryology that pushed Darwin toward the idea of evolution is that the early embryos of most vertebrates closely resemble one another”⁷.

Burton Guttman’s 1999 textbook, ‘Biology’, accompanies its redrawn version of Haeckel’s embryos with the following caption:

 “An animal’s embryonic development holds clues to the forms of its ancestors”³⁵.

Some textbooks, instead of reproducing or redrawing Haeckel's embryos, use actual photos." How dare those nefarious textbook authors use photographic data to support their ideas. Sylvia Mader’s 1998 “Biology”, for example, includes photos of chick and pig embryos, accompanied by this caption:  

“At these comparable early developmental stages the two embryos have many features in common although eventually they are completely different animals.  This is evidence that they evolved from a common ancestor”³⁶.

Wells evaluates the issue as follows:

“Mader's use of actual photos instead of faked drawings is a step in the right direction, but the embryological evidence is still being misrepresented As we have seen, Haeckel's distortions of embryos in mid-development was just one of his misrepresentations; the others were his biased selection of classes and orders that fit his theory, and his omission of earlier stages. Both of these misrepresentations are perpetuated.

Furthermore, although biologists have also known for over a century that Haeckel's drawings are fakes, and that the earliest stages in vertebrate development are not the most similar, textbooks continue to use those drawings (or almost equally misleading photos) to convince unsuspecting students that Darwin's theory rests on embryological evidence”³².

Footnotes:

1. Wells, J. Icons of Evolution, Science or Myth? Trns. Orhan Düz. Evrimin İkonları, Bilim Mi Mit Mi? Gelenek yayıncılık. Kurtiş Matbaası, İstanbul, 2003.

2. Henfrey, A. and Huxley, H.T. (Editors). Scientific Memoirs: Selected from the Transactions of foreign Academies of Science and from Foreign Journals: Natural History. London, 1853 (Reprinted, 1966 by Johnson, Reprint Corporation, New York).

3. Şengün, A. Evrim. Sermet Matbaası. Kırklareli. 1984, s.142.

4. Richard, M. Shattering the Myths of Darwinism. Trns. İ. Kapaklıkaya. Son Tartışmalar Işığında Darwinizm’in Mitleri. Gelenek Yayıncılık, İstanbul, 2003

5. Gish, D.T. Evolution: The Fossils Say No! 1981. Trns. Â. Tatlı,  Fosiller ve Evrim. Cihan Yayınları, İs­tanbul. 1984.

6. Curtis, H. and Barnes, S. Invitation to Biology. 5^th impression, New York: Worth Publishers, p.405, 1994.

7. Gould, J. L. and Keeton, W. T. Biological Science. 6^th impression, New York: W. W. Norton, p. 347, 1996.

8. Raven, P. Johnson, G. Biology.5 the impression, Boston: WCB/ McGraw-Hill, p. 416, 1181, 1999.

9. Futuyma, D. Evolutionary Biology. 3^rd impression, Sunderland, MA:Sirauer Associates, p.653, 1998.

10. Rager, G. Human Embryology and The Law of Biogenesis. Rivista di Biologia 79, p. 449-465, 1986.

11. Denton, M. Evolution. A Theory in Crisis. Burnett  Books, London, 1985.

12. White, A. J. Wonderfully Made. Evangelical Pres, Darlington.

13. Guyer, M.F. Animal Biology. New York. Harper; Brothers, London. 1937. 

14. Lenoir, T. The Strategy of Life. The University of Chicago Press, p.258, 1982.

15. Sedgwick, A. The Influence of Darwin on The Study of Animal Embryology, p.171-184. A.C. Seward (Editor). Darwin and Modern Science. Cambridge University Press, p.174-176,1909.

16. Lillie, F. R. The Development of the Chick. 2^nd impression, New York, Henry Holt, 1919.

17. Rasmussen, N. The Decline of Recapitulationism in Early Twentieth Century Biology. Disciplinary Conflict and Consensus on the Battleground of Theory. Journal of the History of Biology, 24, p.51-89, 1991.

18. Beer, G. Embryos and Ancestors. 3^rd impression, Clarendon Press, Oxford, p.10, 164,172, 1958.

19. Oppenheimer, J. M. Haeckel’s  Variation on Darwin. S. 123-135. Heonigswald, H. M. and Wiener, L. F. (Editors), Biological Metaphor and Cladistic Classification. University of Pennsylvania Press, Philadelphia,  p.134, 1987.

20. Richardson, M.K. Heterochrony and  The Phylotypic Period. Developmental Biology 172, p.412-421, 1995.

21. Richardson, M.K., Hanken, J. Gooneratne, M. L., Pieau, C, Raynaud, A., Selwood, L. and Wright, G.M. There is no Highly Conserved Embryonic Stage in The Vertebrates: Implications for Current Theories of Evolution and Development. Anatomy and Embryology, 196, p.91-106, 1997.            

22. Richardson, M.K., Allen, S. P., Wright, G.M., Raynaud, A. and Hanken, J. Somite Number and Vertebrate Evolution. Development 125, p.151-160,1998.

23. Gould, S. J. Atrocious! Natural History, March, p.42-49,2000.

24. Beer, de G. Embryos and Ancestors. 3^rd impression, Oxford: Clarendon Press, p. 152, 1958.

25. Hitching, F. The Neck of the Giraffe: Where Darwin Went Wrong?. New  York: Ticknor and Fields, 1982, p.204.

26. Keith, S.T. Ontogeny and  Phylogeny Recapitulated. American Scientist, Volume 76, May/June, 1988, p.273.

27. Ballard, W. W. Problems of Gastrulation: Real and Verbal. Bioscience 26, p.36-39, 1976.

28. Blechschmidt, E. The Beginnings of Human Life. Translated by Transemantics. Spring-Verlag, New York, p.29-30,1977.

29. Elinson, R. P. Change in Developmental Patterns: Embryos of Amphibians With Large Eggs. S.1-21. R.A. Raff and E.C.Raff (Editors). Development  as an Evolutionary Process, Volume 8, New York, p. 3,1987.

30. Balinsky, B.I. An Introduction to Embryology. 4^th impression W.B.Saunders Company, Philadelphia, p.7-8, 1975.

31. The digital City-Kansas City Public Evolution, 17 February, 2000. Board:http://home.digitalcity.com/kansascity.

32. Wells, J. Icons of Evolution, Science or Myth? Trns. Orhan Düz. Evrimin İkonları, Bilim Mi Mit Mi? Gelenek Yayıncılık. Kurtiş Matbaası, İstanbul, p. 107, 2003.

33. Alberts, B., Bray, D., Lewis, J. Raff, M. Roberts, K. and Watson, J. D.  Molecular Biology of Cell. 3^rdimpression, New York: Garland Publishing, p.32-33, 1994.           

34. Star, C. and Taggart, R. Biology: The Unity and Diversity of Life. 8^th impression, Belmont, CA: Wadswoth Publishing Company, p.317, 1998.

35. Cloud, P. Pseudofossils, A Plea For Caution. Ge­ology. November. 1973, p. 123.

36. Mader, S. Biology. 6^th impression, Boston: WCB/ McGraw-Hill, p. 298, 1998.

37. Simpson, G.G. An Introduction to Biology. New York, Harcourt Brace and World, 1965, p.241.