After the fertilization when the zygote is in the growing state to a foetus, the main canal is introduced first, from mouth to anus. But my question is when the canal is created first how does it determine which side is for mouth and which side is for anus?

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    $\begingroup$ This question reminded me of the fact that humans are assholes $\endgroup$ May 22, 2015 at 8:32
  • $\begingroup$ From my understanding, the location of the initial fold (blastopore) during gastrulation always becomes the anus (in deuterostomes, e.g. humans). I don't know how the polarity of the primitive streak is formed though. $\endgroup$ May 22, 2015 at 8:38
  • $\begingroup$ You have enough rep now to upvote mate. $\endgroup$
    – Rover Eye
    May 22, 2015 at 10:18

1 Answer 1


The gut tube is formed from endoderm that migrates through the primitive streak. Before this, however, the anterior-posterior axis of the embryo is defined by the formation of the streak at the beginning of gastrulation. In mice, which are phylotypic with humans at this stage, the streak forms posteriorly and then extends to the anterior. I'm not sure if you'll be able to find much research on humans, but mice and chicks are quite well studied. That said, I don't know the molecular and cellular mechanisms which define where the primitive streak forms. I don't even know if it is known. How much detail are you looking for? If I find the time, I can try and elaborate on this further. In the mean time, this review looks promising:


  • $\begingroup$ Thank you so much for the answer. Actually as much research paper and books I studied so far didn't get any particular answer for my question. I will will statisfied if I get to know the determination of position of anterior and posterior axis in which basis. I mean if an anterior and posterior axis is chosen for the purpose then can the place where anterior is present could be for posterior position? I heard from my teacher that it's determined by a particular protein condrin. Though couldnt understand the answer properly. $\endgroup$
    – Demietra95
    May 22, 2015 at 6:24
  • $\begingroup$ And the link is good and sorry couldn't vote up the answer as dont have enough reputation, just joined today. $\endgroup$
    – Demietra95
    May 22, 2015 at 6:26
  • $\begingroup$ apparently polarity is already present in the gametes. I tried to formulate an answer but gave up because of the complexity. $\endgroup$
    – AliceD
    May 22, 2015 at 6:40
  • $\begingroup$ @AliceD Is that true of amniotes? Maternal effect is quite contentious in this class. $\endgroup$
    – canadianer
    May 22, 2015 at 7:03
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    $\begingroup$ This wiki-section should be relevant to your answer: Wikipedia: Gastrulation - Formation of the primitive streak, and it contains references to a couple of signalling pathways and protein involved. $\endgroup$ May 22, 2015 at 8:35

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