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My textbook says that regeneration isn't possible in complex multicellular organisms.

But why? Why can't specialized cells be present in the bodies of complex multi cellular organisms which can proliferate and ultimately make a whole new organism, when (for example) cut in two?

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    $\begingroup$ What textbook are you using? Regeneration most assuredly is possible in many complex multicellular organisms, such as the Asteroidea of the Echinoderms. $\endgroup$ – rotaredom Feb 7 '19 at 18:02
  • $\begingroup$ The book means to say in most organisms $\endgroup$ – NightKruger Feb 7 '19 at 19:32
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I was doing some research on this myself and from what I had found, the body has hox genes that direct the formation of body parts. So now the obvious question is that if a body part contains stem cells and hox genes, shouldn't it be able to regenerate a new body? Well, perhaps. But the problem is that as organisms become more complex, the process of the creation of their bodies also becomes more complex. For example, in the fruitfly, if the labial gene function is not expressed, the mouth and head that develop outside the body do not get involuted and that prevents the salivary gland from forming.

So the answer about why multicellular organisms can't regenerate so easily simply seems to be because the process of creating a multicellular body has for some reason become an extremely complex, intertwined process where various body parts creation is dependent on other body parts creation. So in the evolutionary process it's entirely possible that the stem cells noted that there's no point trying to regenerate a body part, and therefore they just don't.

Here's an anomaly. There's a jellyfish that can reverse aging and go back to infant stage. I think it's able to do so because it has a simple body structure.

Another possible reason could possibly be the presence of unstructured proteins which are more abundant in multicellular organisms.

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    $\begingroup$ The jellyfish you mentioned is turritopsis dohrni if I remember correctly. When I was young, I wanted to be like that jellyfish. Now I know that im too 'complex' (instead of simple) to be that. $\endgroup$ – NightKruger Feb 7 '19 at 8:49
  • $\begingroup$ An evolutionary reason for the lack of regeneration in long lived animals is cancer. The waiting undifferentiated cells (stem cells) are another potential source of cancer. $\endgroup$ – Polypipe Wrangler Apr 16 '20 at 5:42
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Not all complex organisms are incapable of regeneration!

The most famous example is probably the axolotl - a salamander that is able to regrow whole limbs or even parts of essential organs like the brain or heart. While it's not yet know how that is possible, there are a lot of researchers looking into it (the genome of this species was fully sequenced last year).

Another example is the regeneration of zebrafish fins, which - while less impressive - is by now pretty well studied.

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