VMartin
Posts: 525
Joined: Nov. 2006
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John Davison has written in his Manifesto:
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The actual facts are as follows. In birds the cells destined to become the germ cells first appear in the extra-embryonic endoderm (germinal crescent) anterior to the head of the developing embryo.
Incidentally, this region has no homologue in the hatched bird as the extra-embryonic endoderm is, by definition, resorbed as nutrient for the developing chick. From there the presumptive germ cells enter the circulatory system and, after a period of time in the bloodstream, penetrate the walls of the venous circulation and invade the gonad where they differentiate into the definitive gametes. In mammals the presumptive germ cells first appear in the endoderm of the allantois, a structure destined to become the urinary bladder of the adult. From here they migrate in amoeboid fashion anteriorly and laterally to reach the gonad where they complete their differentiation. Thus, there is no way that the reproductive cells of mammals can be homologized with those of birds as they originate from opposite ends of the embryonic axis and reach the gonads by completely different means.
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Adolf Portmann adressed almost the same problem from another point of view. Because all of you are excellent linguists I suppose you have no problem reading German. I quoted from Adolf Portmann's "Biologie und Geist - Die Biologie und das Phanomen des Geistigen", Zurich 1956, page 22:
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Verfolgt man nun in dieser naturlichen Serien von Tiertypen die Lage der mannlichen Keimdrusen, dann stellen wir fest, dass diese Hoden in der aufsteigenden Reihe vom Fisch zum Sauger aus der vorderen Rumpfzone oder der Rumpfmitte mehr becken-, ja leistenstandig werden, um schliesslich in einem Hodensack ausserhalb der Leibsholle in geradezu paradoxer ?Situation am analen Korperpol eingelagert zu werden.
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Whats more important Adolf Portmann didn't see any plausible "selection" explanation of it:
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...dass es keine Theorie gibt, welche das Phanomen des <<Descensus>>, des Hodenabstiegs, erklart. Die Erscheinung ist ist um so beachtenswerter, als keine Moglichket gibt, die Entstehung dieses Gebildes durch Selektion zu erklaren.
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Perhaps the prominent Swiss professor didn't hear about "niche"?
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I could not answer, but should maintain my ground.-
Charles Darwin
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