I keep seeing these images ([like this](https://vivadifferences.com/difference-between-protostomes-and-deuterostomes-with-examples/)) where a dent forms in the blastula, grows inwards and eventually pokes to the other side. And in protostomes, the first opening will become the mouth, while in deuterostomes – the anus. But this is in no way compatible with the explanation of embryonic development in [this video](https://www.youtube.com/watch?v=4YKvVeVMmEE) (and similar resources) talking about the embryonic disc (where is the embryonic disc in the invaginating blastula?) and how it folds over the yolk sac, forms the digestive tract and – the video states it clearly – the mouth opening forms first (week 4 or human gestation), the anus – second (7th week). I don’t know how to reconcile all this. Is what is described in these images as “future anus” actually refer to the connection with the yolk sac?