From: bk186@cleveland.Freenet.Edu (Julie Thomas)
Subject: Re: More on three and four chamber hearts
Reply-To: bk186@cleveland.Freenet.Edu (Julie Thomas)
Organization: Case Western Reserve University, Cleveland, OH (USA)
Date: Tue, 14 Apr 92 01:53:25 GMT


In a previous article, salem@pangea.Stanford.EDU (Bruce Salem) says:

>Did you see my posting from a couple of days ago in the
>developmental sequence of the four chambered heart in
>humans and its relation to the anatomy of two and three
>chambered hearts?

Yes I did.  Thank you.  However, I did not find it to be all that
helpful.  If I recall, you posted about the human heart developing
from a "primitive" structure into the four-chambered heart in a manner
somewhat suggestive of the evolutionary procession.  But unless the
null hypothesis would be that the heart should develop in a different
manner, I fail to see how this is evidence of the supposed transformation.
Are you suggesting that without evolution, we should expect, for example,
the heart to develop from some 12 chamber stage?  And considering that
development proceeds from a single cell towards a complete multicellular
organism, I would expect earlier structures to be less defined.  



>That account (Arey, 1942 Developmental Anatomy pg. 316 ff)
>sayes that the development proceeds in stages roughly analogous
>with the enchancements from fish to mammalian form.

Right.  "Roughly analogous".  That does not give me reason to suspect
this must reflect some ancient phylogenetic transformation.  


 The heart
>begins as a thichening in vascular structures in the crainial
>area of the embryo, develops into a three chambered affair
>with atruim, ventricle, and bulbus.

Well, the fish heart consists of a four chambered affair in series.


 A forth and smaller segment,
>the sinus venoosus, which developed out of a different structure
>than the rest of the heart, unites with the right side of the
>atrium.

If this is true, then the fish heart is most different.  


 As the heart developes the atrial and ventricular partitions
>begin to appear at the same time that the bulbus is absorbed by
>the ventricle. Out of the bulbus both the aorta and pulmonary arteries
>are formed, and this is not due to a partition of the atruim. At
>this point the venous circulation enteres the atrium and the
>arterial leaves from what used to be the bulbus. The heart is
>still three chanbered. It is the transformation of the bulbular
>region and ajoining parts of the right ventricle into the
>right ventricle and the simutaneous fornation of the remaining
>septa that results in the four chambered heart. The partitioning
>occurs after the antrial and venous circulation have become
>isolated at opposites ends of the heart. So the transformation
>of the polmonary function predates in the development, at least,
>the greater efficiency effected by partitioning and formation of
>valves.

How does this help the reptile to mammal heart transformation?
Recall that in the reptile (with a three chambered heart) both
atria enter into the left ventricle.  What you need is the two
atria dumping into the left ventricle, then the right one shifting
to the right as the intervenricular septum completes it's growth.
The funny thing is that when it does, there remains an opening
between atria.  



>	Now, I hasten to add that I an not making a case for
>recapitulation, as Gould makes a good case that the relation
>between development and phylogenetic steps is very much
>more complicated than that.
>

Covering the bases, eh?  I have a great deal of difficulty accepting
ontogeny as evidence for evolution when the cards are stacked so that
similarities supposedly mean relationship and differences are assigned
to some "special" category where ad hoc explanations reign.  The
whole endeavor is set up so that only evidence for evolution can be
found.  I reject this methodology.  If similarities count for,
differences count against.  (of course, similarities might not count
for and differences might not count against; subjective interpretations
are everywhere!).
 
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