Now in Japan

                 I only put the covers of the new volumes beacause I don't want to know much about them, I prefere wainting for the french publication and keep the suspense intact

For impatients ones here a two websites where you can find english translation of each prepublished chapter

http://www.mangascreener.com/project.php?id=8 

http://stephen.swoo.net/20cb/20cb.html 

                            How does it work ? everything about prepublication (Thanks to Jipé for the writting and the english translation ^_^ )

                                                                                                                                        Prepublication magazines

There's a great deal of them in Japan. Published weekly, twice or nce in a month, the majority of the content is manga series as well as mangaka interviews, articles on various topics of interest for the japanese, and girls dressed in swimsuit when the targeted readers are adults.
These magazines apparently don't make any money but make a lot of advertising for the manga that are subsequently published by assembling the prepublication chapters (tankobon).
The most popular magazine is Shônen Jump from the Shueisha editor which targets adolescents and has weekly sales of approximately 3 millions. The series published in this magazine are tremendous successes, namely One Piece, Bleach, Naruto, Eyeshield 21, Deathnote, Hunter X hunter, Bobobo bo bo bobo.
The magazines targeting an older audience realise lower sales but the most popular reach a million nevertheless (for example Young Jump where Gantz, Zetman, Elfen Lied are published)

                                                                                                                        Naoki Urasawa manga prepublication

Naoki Urasawa is bound to an editor, Shôgakukan and I believe that all his works have been published by this editor. The chapters from his series have been published in the various magazines of his editor :

Pineapple army was published in Big Comic Spirits
Master was published in Big Comic Original
Yawara was published in Big Comic Spirits
Happy! was published in dans Big Comics
Monster was published in Big Comic Original
Pluto is published in Big Comic Original
20th Century Boys is published in Big Comic Spirits

                                                                                                                                        Big Comic Spirits


Link to the cover and index of the magazine of the week : http://spi-net.jp/this_week/index.html

20th Century Boys is apparently the big title published in Big Comic Spirits. It sometimes make the cover (often to announce the release of a tankobon but most of the times it's a sexy japanese girl that is on the cover) and there are rarely color pages for the magazine series, therefore there are rarely color pages for 20th Century Boys.
Examples of covers :

http://www2.ocn.ne.jp/~azuma1/boy56.files/1.JPG
http://www2.ocn.ne.jp/~azuma1/boy74.files/1.jpg
http://www2.ocn.ne.jp/~azuma1/boy76.files/sp.JPG
http://www2.ocn.ne.jp/~azuma1/boy82.files/sp.JPG

Sometimes there are exclusive stuff in the magazine, such as contests to win rarities or stickers :

http://www2.ocn.ne.jp/~azuma1/boy57.files/1.JPG

Having bought the magazine twice (it can be bought in Paris at Junku, 6€ to be compared with 270yen in Japan) it can be indicated that the global level of the manga series force is not very high. Indeed, as 400 pages need to be filled each week, very few ambitious series are in there and we find the habitual cooking, sports, erotic series. That easily explains why these series are dropped in dustbins once the major series have been read.

Some series published in Big Comic Spirits have been distributed in France, namely Homunculus, Maison Ikkoku, She the ultimate weapon.

Thanks to the forum of Mangaverse for the figures. Thanks to the site  http://www2.ocn.ne.jp/~azuma1/boyboy.htm for the images, great site if I could read japanese.

By the way on this site, I found a photo that has probably inspired Urasawa for the Guts bowl : http://www2.ocn.ne.jp/~azuma1/boy32.files/tate.JPG