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#1 PhilC

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Posted 06 July 2010 - 03:31 AM

Here is an image of how the circulation system in the vertebrate embryo looks. I’ve drawn this myself based in images from a book Biology A functional approach M B V Roberts 1986 Nelson. If anyone wants to use this image for any work on vertebrate embryo’s then they are welcome. It works for fish, amphibian, reptile, bird and mammal embryo’s. They are all the same.

If your work is about adult fish, this can also be used (though in some adult fish the 1st and 2nd arterial arches sometimes disappear in the adult. The arterial arches are where, in fish, the blood circulation goes to the gills. The hole at the bottom is where the blood goes round the body, so in embryo’s and fish the oxygenated blood is completely mixed with the deoxygenated blood

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In order to get an adult fish circulation system, just use this one that is actually an image of the tadpole system. Once again, in tadpoles and in fish, the arterial arches go to the gills. In actual fact, tadpoles show the first signs of the heart splitting into two sides. This splitting of the heart is seen in all terrestrial vertebrates.

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Here is an amphibians circulation system based on the adult frog. Look back at the embryo of the frog above (first image, remember that all vertebrates start like that, but following development of the tadpole into the frog we see that the arterial arch no. 3 constricts to become the vessels for the head (carotid arteries/veins), no, 4 becomes the circulation for the body (systemic arteries/veins) , no. 5 disappears and no. 6 becomes the circulation that goes to the lungs (pulmonary arteries/veins). The heart becomes separated into two halves. The way the side arches disappear is seen in all terrestrial vertebrates.

The fact that the 6th arterial arch from its pharyngeal position in fishes becomes linked to the heart/lungs leads to one of the strangest pieces of design in nature. But that’s another story for another day.

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Now we get to the crocodile circulation system (a representative reptile, there is some variation in reptile circulatory systems). Remember that this can be seen to develop from the original fish like circulation system at the top of the page. In the crocodile the two blood systems of oxygenated/deoxygenated blood are separate, or almost; there is only a small hole between the two sides of the heart. The left systemic arch (no. 4) carries deoxygenated blood. Notice that the left systemic arch (no. 4) comes from the right ventricle. This will become important later. The hole in the heart is seen in human embryos but it normally closes when the child is born.

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Here is the bird circulation. It has the same setup to the crocodile one, but the left systemic arch has disappeared and there is no hole in the heart. This means only oxygenated blood goes round the body, a much more efficient system.

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#2 PhilC

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Posted 06 July 2010 - 03:33 AM

I've had to split this to show all the diagrams, sorry.


Finally another efficient system, but one that is the reverse of the bird one. The right systemic arch has disappeared and we have the mammalian system. Again, no hole in the heart and only oxygenated blood going to the body.

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This difference between the bird and mammalian circulation was one of the main pieces of evidence for showing that reptiles and birds are closer together than mammals and birds. This was known before Darwin.

#3 PhilC

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Posted 09 July 2010 - 03:30 AM

Baer's laws:

1.General characteristics of the group to which an embryo belongs develop before special characteristics.
2.General structural relations are likewise formed before the most specific appear.
3.The form of any given embryo does not converge upon other definite forms but, on the contrary, separates itself from them.
4.Fundamentally, the embryo of a higher animal form never resembles the adult of another animal form, such as one less evolved, but only its embryo.


http://en.wikipedia....28embryology.29

Von Baer described and Darwin explained.




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