A term for the Japanese syllables represented in the fifty sounds chart. The chart itself is a five by ten matrix of syllables. The columns are ordered from right to left, and the sounds in each column are ordered from top to bottom. While the kana ん (cf. syllabic N in Japanese) exists in the syllabary, since it isn't in the chart, it's not considered one of the fifty sounds. Also, despite being called the fifty sounds chart, it never contained fifty unique sounds in the chart.
The modern 50 sounds chart in hiragana

わ ら や ま は な た さ か あ
  り   み ひ に ち し き い
  る ゆ む ふ ぬ つ す く う
  れ   め へ ね て せ け え
を ろ よ も ほ の と そ こ お

Tongpoo's romanization *

 wa  ra  ya  ma  ha  na  ta  sa  ka  a
     ri      mi  ci  ni chi shi  ki  i
     ru  yu  mu  fu  nu tsu  su  ku  u
     re      me  he  ne  te  se  ke  e
wo/o ro  yo  mo  ho  no  to  so  ko  o

pronounciation key **

letter   example                     linguistic term              IPA
  c      h in heat                   unvoiced palatal fricative    ç
  f      wh in which                 unvoiced bilabial fricative   φ
  r      t in water, Brooklin accent alveolar tap/flap             ɾ
  u      like blowing out a candle   close back unrounded vowel    ɯ
* The above romanization scheme I just made up. For common romanization schemes of the Japanese language, see: Japanese romanization.

** Sounds not mentioned in the pronounciation key sound like what they look like. Today, 'wo' is most often pronounced as 'o' instead. A survey (http://nhg.pro.tok2.com/reserch/reserch1-5.htm) suggests about a third of the population of Japanese speakers are pronouncers of 'wo.'

The early 50 sounds chart

わ ら や ま は な た さ か あ
ゐ り い み ひ に ち し き い
う る ゆ む ふ ぬ つ す く う
ゑ れ え め へ ね て せ け え
を ろ よ も ほ の と そ こ お
ワ ラ ヤ マ ハ ナ タ サ カ ア
ヰ リ イ ミ ヒ ニ チ シ キ イ
ウ ル ユ ム フ ヌ ツ ス ク ウ
ヱ レ エ メ ヘ ネ テ セ ケ エ
ヲ ロ ヨ モ ホ ノ ト ソ コ オ
(tongpoo's romanization)
wa  ra  ya  ma  fa  na  ta  sa  ka  a
wi  ri  yi  mi  fi  ni chi shi  ki  i
 u  ru  yu  mu  fu  nu tcu  su  ku  u
we  re  ye  me  fe  ne  te she  ke  e
wo  ro  yo  mo  fo  no  to  so  ko  o
The above chart resembles the earliest 50 sounds chart known, which developed during 11th century Heian era Japan. Until the 20th century, Japan wrote everything from right to left. The chart is thought to have been developed by buddhists who knew the works of an Indian linguist Siddham. Eventually, the more organized 50 sounds chart replaced the iroha (developed around 10th-11th century) in popularity as the preferred order of syllables.

The ha column

The modern version of this column has a non-homogenous set of consonants. The evolution of this column has been:
  1. pa→fa→ha
  2. pa→fa→wa
  3. pi→fi→hi→ci (palatization by "i")
  4. pu→fu
  5. pe→fe→he
  6. po→fo→ho
Where the f + (vowel) stage lasted mostly during the nara era (710 - 780 A.D.). The transition from 'p' to 'f' was from an unvoiced bilabial plosive to an unvoiced bilabial fricative. By the edo era, the column has changed into mostly h's. Okinawan dialects still often preserve the 'p' sounds. As you see in item 2, in modern days は('ha') is sometimes pronounced as は('wa').

The a, wa, and ya column

  1. e/ye→ye
  2. ye/we→ye→e
  3. i/wi→i
  4. o/wo→wo
As you see here, while ゐ('wi') changed to ゐ('i') directly, ゑ('we') changed to ゑ('ye') before becoming an 'e' sound.

The dropping of ゐ and ゑ

Noting that pronounciation differences between ゐ and い, ゑ and え doesn't exist anymore, the government ordered in 1946 to stop the use of letters ゐ and ゑ. In 1986, the rules were relaxed to allow for historical usage of these characters.

All of my sources are in Japanese.
Copies of the orders to drop ゐ and ゑ can be found here: http://www.asahi-net.or.jp/~lf4a-okjm/genkan61.htm
I obtained most of the information for this w/u from: "日本語の起源" http://www.biwa.ne.jp/~ichhan-j/index.htm