Where could I find the proclamation of the dogma ... I assume thats where the rationale for the belief is laid out ... ??
Well, sorta. That's where the official statement is, but the rationale sort of precipitates out of centuries of conversation and some disagreement -- including misunderstandings of embryology(!)
Here is the Encyclical Ineffabilis Deus
Here is the outline of the thinking in the Catechism.