Ok, a first step. We agree that we are talking about human embryos. Very good. Now let’s look a bit at the photo sequence which pictures the developmental stages of an human embryo. Therefore please open this link , look for the sentence “At around the 28th day“ and click on the link next to "day", named <10>. You‘ll see three pictures of an human embryo in the Carnegie stages 10, 11 and 12 corresponding to day 28, 29 and 30.
Do we agree that the embryo is almost straight on day 28 and 29? Do we agree that it get its “C” shape on day 30?
The link says it is nearly straight at the beginning of the week, not the end, and in any event, it seems the biomechanical analysis, on a more detailed look, involves the rate of growth for particulars in that area, not just the whole embryo. So presumably the folds can occur when the embryo is straight.
Given that the rate of growth is governed by gene exression this appears to be conceding Bernd's position rather than arguing against it.
In support of your contention, that the embryo is not neccessarily curved when the first pharyngeal/visceral arches appear, the same section says...
Cervical and caudal flexures have the effect that the embryo takes on its characteristic C shape.
So the embryo has begun to curve by day 28.
There is a higher detail image of day 29 on the side of the web page where it says 'picture' on the second page covering the fifth week. This highlights the developing arches and the begining of the head/neck flexion is clearer. The day 30 embryo picture gives a very clear view of the developing arches and I really think it would be hard to make a case for such protuberant structures being the result of the flexion of the head and neck. These folds are almost as wide again as the neck itself.
However, I am not an embryologist, but the professor dismissing gill pouches does so for a reason, it seems to me, and so I am still wondering exactly why you dismiss the concept of tension here.
Because other than Blechshmidt's word for it there is no data to base a conclusion that tension is the source.
This all seems rather academic if you are prepared to concede, as you stated earlier, that it is simply a question of nomenclature for you and that similar patterns of gene expression and gene function in no way affect your preference for 'biomechanical flexion folds' as the correct term and that the terminology does not have any relevance to the question of homology.
If you are prepared to accept that these genes can control the development leading to the formation of the 'biomechanical flexion fold' then you fundamentally diagree with Blechshmidt's approach to embryology, even if you prefer his terminology in this instance.
This message has been edited by Wounded King, 19-Dec-2005 11:15 PM
This message has been edited by Wounded King, 20-Dec-2005 12:05 PM
Your right with your critic concerning the “C” shape. I’m not sure whether a more exact description - for example slightly curved, instead of almost straight for day 29, or even better to indicate that the flexion starts with day 28 - would have made a big difference for the argument, but I concede that one should try to formulate as exact as possible. This I clearly did not! Anyway, thanks for the correction!
It seems to me that in your last post you have stopped to claim that flexion is the cause for the development of pharyngeal arches, instead you seem to opt for tension. Do I understand you correctly? If yes, that would be - after your concession that development is under genetic control - the second mayor point where you contradict Blechschmidts ideas.
But whether you propose flexion or tension as mechanism, in both case you have to deal with the data, therefore I would ask you to explain in some detail how the sequence in which the pharyngeal arches appear and disappear could possibly be explained by “tension” or “flexion”. (for a description of this sequence have a look at )
Lets analyze now, whether it makes sense to call the arches “folds“. I am going to raise two objections, first, the term “fold” suggests something simple. In reality the pharyngeal arches are structured . Each contains a nerve, an aorta, cartilage (compare  ). Would we - to bring up a similar case - tend to call the human head a “fold” because it’s first appearance is nothing more than that? If we accept this argument we would restrict the usage of “fold” to the stages where the arches are nothing more than a swelling. But when we look at this stage, we note that the term “fold” - understood as flexion or tension fold - does not describe accurately how the arches develop. Lets have a look at  which describes the appearance of the first two arches at Carnegie stage 10 ( day 28):
Swellings appear on the external part of the craniofacial region of the embryo that demarcate the first (mandibular) and the second (hyoid) branchial (pharyngeal) arches. The swelling is due to invasion of the area by neural crest and multiplication, in situ, of existing mesoderm-derived mesenchyme.
(You’ll find some background in , specifically have a look at the following diagram )
Considering this information, can we agree that neither “flexion” nor “tension folds” are adequate terms for pharyngeal arches?