This is not correct. At no point can the offspring in a single generation be differnet enough to be called a different species.
I'm not saying we should call it a different species but if we're saying species Y is the direct descendant of species X, then, we can imagine a dividing line, and the line must always begin with an egg because eggs are different from their parents but adults are not different from the egg they started off as.
In reality things change very slowley over a large amount of time and there a no clear transition points.