A mesostic poem is a form of inclusion poetry, or acrostic poem in which the "hidden" or included word, phrase, or name is seen vertically in a central spine instead of at the beginning or end (or both for a "double acrostic") of each line. The form was popularized (if such a thing is possible with such esoteric poetry) by John Cage, who used the term to describe a particular form of inclusion poetry that had compositional rules beyond simply holding the inclusion word or phrase down a central spine. Since the term was first used to by Cage to describe his particular type of meso-acrostic, there is some debate in literary and artistic circles about what can be properly called a "mesostic".
My own opinion is that the term can rightly be applied to any acrostic-type text composition in which the inclusion runs down the spine as opposed to the edge. This opinion is based on the perception that Cage was writing a particular type of specialized mesosic and not the only type of mesostic, even though the term was used mostly to describe his particular central acrostics.
Cage said that, "Like acrostics, mesostics are written in the conventional way horizontally, but at the same time they follow a vertical rule, down the middle not down the edge as in an acrostic, a string spells a word or name, not necessarily connected with what is being written, though it may be. This vertical rule is lettristic and in my practice the letters are capitalized. Between two capitals in a perfect or 100% mesostic neither letter may appear in lower case. .... In the writing of the wing words, the horizontal text, the letters of the vertical string help me out of sentimentality. I have something to do, a puzzle to solve."
From what I have been able to glean hither thither and yonder in Netland there are with three variations of the acrostic form which are called "mesostic".
I'll begin with the two forms for which Cage and writers/artists/poets following his lead seem to be in total agreement:
1) The 50% Mesostic (A given letter capitalized does not occur between it and the preceding capitalized letter)
2) The 100% Mesostic (A given letter capitalized does not occur between it and the preceding or following capitalized letter)
3) The basic form (possibly the original form - but not for Cage) is the one on which there seems (at least to me) to be some ambiguity and some room for interpretation.
This might be termed the proto-mesostic, early mesostic, quasi-mesostic or neo-mesostic, depending on where one places it conceptually or chronologically. In one article (see the references below) about Cage and his acrostics/mesostics the basic form was accepted by Cage as adequate to be described as a mesostic. In this article Cage showed a colleague some "acrostic" poems that he had written in which the acrostic word/phrase was arranged vertically down a central spine rather than 'bookending' each line at the beginning or end (or both as in a double-acrostic). Cage termed these poems as 'mesostics', describing an acrostic form with the the phrase in the middle (hence 'meso'). He later went on to refine and redefine the form into the 50% and 100% mesostic.
So the issue is, if only the two forms used by cage can properly be termed "mesostic", what does one call the "ordinary mesostic"? It is not a typical acrostic. It is not 50% mesostic, nor is it a 100% mesostic. I remain wont to call it a mesostic as that term seems well-suited to describe an acrostic poem with the phrase down the middle.
From Perloff's article (referenced below) it seems that Cage may not have written "0% mesostics" or meso-acrostics himself. But the same article (same paragraph) does suggest that the term "mesostic" may still apply to meso-acrostic poems that do not conform to Cage's mesostic rules, since it may have been Norman Brown and not Cage who coined the term.
"My first mesostic," Cage writes in the Foreword to M, "was written as prose to celebrate one of Edwin Denby's birthdays. The following ones, each letter of the name being on its own line, were written as poetry. A given letter capitalized does not occur between it and the preceding capitalized letter. I thought I was writing acrostics, but Norman O. Brown pointed out that they could properly be called 'mesostics' (row not down the edge but down the middle)" (M 1).
Create your own instand mesostic at:
http://www.euph0r1a.net/ (website of Matthew McCabe, a Florida PhD student and composer/geek) he lists the following in his bibliography (http://www.euph0r1a.net/mesostomatic/biblio.html):
John Cage Computer Programs
(Andrew Culver, author. Date of authorship unknown. Accessed May 21, 2006).
Available at: http://anarchicharmony.org/People/Culver/CagePrograms.html
The Music of Verbal Space: John Cage's "What You Say"
(Marjorie Perloff, author. Date of authorship unknown. Accessed November 18, 2003).
Available at: http://www.ubu.com/papers/perloff02.html