Part of the axoneme consisting of a highly electron-dense region at the distal end of the ciliary transition zone within the axonemal lumen at which the axonemal central pair of microtubules is connected to the rest of the axonemal structure. [ http://www.ncbi.nlm.nih.gov/pubmed/23352055 http://www.ncbi.nlm.nih.gov/pubmed/4554367 ]