Short answers, pulled from the story.
Walther Flemming observed a central structure in cells during 1875. He published his findings in Studien uber die Entwicklungsgeschichte der Najaden that same year.
A typical centriole forms nine sets of short microtubule triplets arranged in a cylinder. This cylindrical shape relies heavily on the protein tubulin for its main structure.
Crabs and Drosophila melanogaster embryos contain nine doublets instead of triplets. Caenorhabditis elegans sperm cells and early embryos feature nine singlets of microtubules.
Land plants do not contain centrioles except within motile male gametes. Conifers and flowering plants completely lack centrioles in all their cells.
Meckel, Gruber syndrome connects directly to this migration failure. Specific genetic disorders arise when centrioles fail to migrate properly before assembly.