T. Thomson Flynn and the monotreme egg from oocyte maturation to germ layer formation

Knowledge of oocyte development and the early differentiation germ layers in monotremes stems largely from two articles by J. P. Hill T. Thomson Flynn. The completeness their account was due to large series echidna ovaries eggs collected on Tasmania Flynn, an Australian biologist whom a brief is given. A striking finding embryo presence latebra connected yolk bed beneath germinal disc as described birds several reptiles. further resemblance vitellogenesis striate zone zona pellucida. Cleavage resulted first lens-shaped blastodisc encircled cells called vitellocytes that later fused form germ-ring. gave rise blastoderm eventually became unilaminar but comprised cell types identified ectoderm primitive endoderm. Eventually these segregated into formed blastula. This sequence resembled marsupials except there no obvious distinction between future embryonic extra-embryonic regions. As extended over surface yolk, it preceded germ-ring played role forming navel. unique feature monotreme development. It shown Flynn important analysis material well its collection.