I have posted a one-page description of the philological sources of the zhùyīn fúhào 注音符號 or bopomofo ㄅㄆㄇㄈ.
The Mandarin Phonetic Symbols (zhùyīn fúhào 注音符號; informally known as bopomofo ㄅㄆㄇㄈ) were selected from among various ancient characters, most of which are no longer in use and some of which are known only from medieval dictionaries that contain all sorts of bizarre and otherwise unknown forms. The Symbols were inspired by the Japanese kana and modeled on an earlier system produced by the eminent philologist Zhāng Tàiyán 章太炎 (1868–1936). The use of ancient forms exhibits a clear nationalist sensibility.
The pronunciations of those characters are shown in the document, together with notes on their meanings and origins. This information is mainly based on the preface to the Guóyīn Zìdiǎn 國音字典 of 1919.