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?
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The formation of the primitive streak is the part you’re interested in. Note that it forms from the caudal (butt) end. The next segment is talking about the rest of gastrulation, with the formation of the germ layers.
In deuterostomes (us), that invagination will become the caudal end and therefore the butt. In protostomes, that invagination will become the cranial end and therefore the mouth.
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