It might be indeterminate whether conjoined twins are one organism or two organisms, but in mythology we find creatures such as the dragon Ladon which are multi-headed by nature. In those fictional cases we definitely have multiple consciousnesses in one organism, because one organism contains more than one brain. (Whether such animals could really exist is another question.)

I'll give this one last shot.

Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially. The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together.

http://en.wikipedia.org/wiki/Conjoined_twins#Separation

So, part of a single embryo splits away, grows in parallel and re-merges with the other part.
One becomes two becomes one.

The one body (embryo) spits into two parts (cf. you and your arm)
The two parts develop independently (cf. hypothetical regenerating arm)
The two parts fuse (cf. your arm reattached, of the same flesh, into the one body)

Can you tease out why one real case of one body splitting and fusing is Teuton's 'two overlapping bodies' (conjoined twins) where the other is one body (you with arm reattached)?

This has fuck-all to do with real arms regrowing.

The fission case seems simpler. The one body never splits, it simply grows two sets of body parts that are always connected.
Here we could compare conjoined twins with extra limbs or organs. One embryo that develops with four legs is one biological individual but one embryo that develops four legs, four arms, two hearts, two head and four lungs is conjoined twins. Where are you going to draw the line?

Teuton wrote:It might be indeterminate whether conjoined twins are one organism or two organisms, but in mythology we find creatures such as the dragon Ladon which are multi-headed by nature. In those fictional cases we definitely have multiple consciousnesses in one organism, because one organism contains more than one brain. (Whether such animals could really exist is another question.)

You are really going to upset DavidMcC with that!

The conjoined twin case is one contiguous autonomous body with (almost) two brains.

Don't you count the mythological creature as multiple overlapping bodies? Wy would you count that as one organism if you don't consider the twin as one organism?

Do we just count heads and tails? One head two tails = two bodies.
Two heads one tail = one body.

"organism n. any living thing."

(Lawrence, Eleanor, ed. Henderson's Dictionary of Biology. 14th ed. Harlow: Pearson, 2008. p. 464)

This oversimple definition is certainly inadequate. For parts of organisms such as organs, tissues, and cells are living things too, but they aren't organisms. And if organisms are living or alive by definition, then there are no dead organisms. But the concept of a dead organism is not a contradiction in terms. An organism doesn't cease to exist when it dies (unless the cause of its death is an explosion which destroys it completely.) We (at least those of us who aren't vegetarians) eat dead animals, but if animals are alive by definition, then what are we eating?
One could take this point into account by using the following definition: Something is an organism iff it lives or has lived.

GrahamH wrote:...
The conjoined twin case is one contiguous autonomous body with (almost) two brains.

No, it's better to see it as conjoined twins, with partially merged brains as well as partially merged bodies.

Don't you count the mythological creature as multiple overlapping bodies? Wy would you count that as one organism if you don't consider the twin as one organism?

I don't count them as anything, as they don't really exist.
______
I regard these twins as neither fully two organisms nor one, for reasons I have already explained. "Ordinary" conjoined twins (ie, those that have entirely separate minds) are clearly two organisms joined together, and, when possible, they are separated by surgeons.

Teuton wrote:"organism n. any living thing."

(Lawrence, Eleanor, ed. Henderson's Dictionary of Biology. 14th ed. Harlow: Pearson, 2008. p. 464)

This oversimple definition is certainly inadequate. For parts of organisms such as organs, tissues, and cells are living things too, but they aren't organisms.
...

True, but there are better definitions than that, involving stipulating a capcity for independent life. Organs and the cells of multicellular organisms are not capable of indendent life.

GrahamH wrote:I'll give this one last shot.

Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially. The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together.
http://en.wikipedia.org/wiki/Conjoined_twins#Separation

Okay, the fission theory supports the one-organism thesis, and the fusion theory supports the two-organisms thesis. Now the question is which one is accepted by the majority of experts in developmental biology.

GrahamH wrote:The fission case seems simpler. The one body never splits, it simply grows two sets of body parts that are always connected. Here we could compare conjoined twins with extra limbs or organs. One embryo that develops with four legs is one biological individual but one embryo that develops four legs, four arms, two hearts, two head and four lungs is conjoined twins. Where are you going to draw the line?

Aren't there objective differences between the embryonic development of conjoined twins and the embryonic development of animals with additional limbs or organs?

Teuton wrote:

GrahamH wrote:I'll give this one last shot.

Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially. The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together.
http://en.wikipedia.org/wiki/Conjoined_twins#Separation

Okay, the fission theory supports the one-organism thesis, and the fusion theory supports the two-organisms thesis. Now the question is which one is accepted by the majority of experts in developmental biology.

GrahamH wrote:The fission case seems simpler. The one body never splits, it simply grows two sets of body parts that are always connected. Here we could compare conjoined twins with extra limbs or organs. One embryo that develops with four legs is one biological individual but one embryo that develops four legs, four arms, two hearts, two head and four lungs is conjoined twins. Where are you going to draw the line?

Aren't there objective differences between the embryonic development of conjoined twins and the embryonic development of animals with additional limbs or organs?

Do you see my point about re-attached arms? Is the result of the re-joining a divided embryo two overlapping bodies, or one reconnected (and possibly extended) body?

Are there (relevant) objective differences between the embryonic development of conjoined twins and the embryonic development of animals with additional limbs or organs? What are they?

Is an embryo a single organism?

DavidMcC wrote:I regard these twins as neither fully two organisms nor one, for reasons I have already explained.

The number of organisms cannot be 1.5, so what can we say? We can say that owing to the semantic vagueness of "organism", the number of organisms is indeterminate; or we can say, as I do, that conjoined twins are two different but not two distinct/disjoint organisms, i.e. that they are two different but partially identical organisms. (Different things certainly cannot be totally identical.) If "two organisms" means "two distinct/disjoint organisms", then the number of organisms isn't 2 in the case of conjoined twins; but if it means "two different organisms", then the number of organisms is 2.

DavidMcC wrote:

Teuton wrote:"organism n. any living thing."

(Lawrence, Eleanor, ed. Henderson's Dictionary of Biology. 14th ed. Harlow: Pearson, 2008. p. 464)

This oversimple definition is certainly inadequate. For parts of organisms such as organs, tissues, and cells are living things too, but they aren't organisms.
...

True, but there are better definitions than that, involving stipulating a capcity for independent life. Organs and the cells of multicellular organisms are not capable of indendent life.

For a more adequate definition, see: http://plato.stanford.edu/entries/biolo ... mProCluKin

GrahamH wrote:

Teuton wrote:It might be indeterminate whether conjoined twins are one organism or two organisms, but in mythology we find creatures such as the dragon Ladon which are multi-headed by nature. In those fictional cases we definitely have multiple consciousnesses in one organism, because one organism contains more than one brain. (Whether such animals could really exist is another question.)

You are really going to upset DavidMcC with that!
The conjoined twin case is one contiguous autonomous body with (almost) two brains.
Don't you count the mythological creature as multiple overlapping bodies? Wy would you count that as one organism if you don't consider the twin as one organism?

Because there's a difference between an animal species whose individual members are naturally multi-headed due to their genetic code, and an animal species such as homo sapiens whose individual members are not. Among the latter, multi-headed individuals are abnormal, pathological phenomena, whereas among the former they are not. (In the dragon species to which Lagon belongs a one-headed individual would be an abnormal, pathological phenomenon.)

GrahamH wrote::lol:

What's so funny? You can tell the difference between a normally multi-headed animal belonging to a species of multi-headed animals and an abnormally multi-headed animal belonging to a species of non-multi-headed animals, can't you?

GrahamH wrote:Are there (relevant) objective differences between the embryonic development of conjoined twins and the embryonic development of animals with additional limbs or organs? What are they?

I don't know. Any biologists here?

Animavore wrote:

Teuton wrote:By the way, just by speaking of "conjoint twins" we tacitly presuppose that there are two organisms, since one organism cannot be conjoined with itself.

Only if we focus on the "twin" aspect of "conjoint twins". I don't see why the biology of these over-lapping people should be restricted to strict English definitions anyway.

Whether so-called conjoined twins are two organisms or one certainly cannot be decided by an ordinary dictionary, because it's a profound scientific and philosophical question. But if conjoined twins are one animal organism, then "conjoined twins" is a glaring misnomer. However, it's not if a distinction is drawn between organisms and persons, such that "twins" refers to two persons and not to one organism. But this raises the question what persons or subjects are (if they are not animal organisms).

Teuton wrote:

GrahamH wrote::lol:

What's so funny? You can tell the difference between a normally multi-headed animal belonging to a species of multi-headed animals and an abnormally multi-headed animal belonging to a species of non-multi-headed animals, can't you?

That you are arguing around 'normal' biology of mythical creatures as the determinant of your mereology.

I think normal development tells us much less than examination of the actual development. Hence my posts on one-two-one split and merge of genetic clones as having some weight.

I suggest that if your mythical beast could be said to develop by the merging of distinct separate entities from separate origins (six eggs)then it is an 'overlapped' multiple, and if it develops by one embryo splitting into 6 then merging back into one then I'd call it one organism with a complex gestation process.

I think this would apply irrespective of what the 'normal' development might be for the species.

I have now tried many links about Tatiana and Krista, but I stll don't know whether or not they see superimposed images from their respective eyes. One article may have hinted that they can switch betwen what their own eyes see and what their sister's eyes see, but it wasn't clear. Quite frustrating, really.

I suspect they might having something a little like blindsight of the other's eyes. One video showed one reaching for an object only in the other's line of sight. It looked very uncoordinated, like groping in a direction that might be using a general sense of location rather than a clear view.

This might make sense given the connection is in thalamus.

There is evidence[8] that the twins' can see through each other's eyes due to brain conjoining. Their thalamuses are joined.
http://en.wikipedia.org/wiki/Krista_and_Tatiana_Hogan

To put it in a more complex way, recent physiological findings suggest that visual processing takes place along several independent, parallel pathways. One system processes information about shape, one about color, and one about movement, location and spatial organization. This information moves through an area of the brain called the lateral geniculate nucleus, located in the thalamus, and on to be processed in the primary visual cortex, area V1 (also known as the striate cortex because of its striped appearance). People with damage to V1 report no conscious vision, no visual imagery, and no visual images in their dreams. However, some of these people still experience the blindsight phenomenon. (Kalat, 2009)

http://en.wikipedia.org/wiki/Blindsight ... s_involved