Please answer a question about the following article about Dutch language:

The Germanic languages are traditionally divided into three groups: West, East and North Germanic. They remained mutually intelligible throughout the Migration Period. Dutch is together with English and German part of the West Germanic group, that is characterized by a number of phonological and morphological innovations not found in North and East Germanic. The West Germanic varieties of the time are generally split into three dialect groups: Ingvaeonic (North Sea Germanic), Istvaeonic (Weser-Rhine Germanic) and Irminonic (Elbe Germanic). It appears that the Frankish tribes fit primarily into the Istvaeonic dialect group with certain Ingvaeonic influences towards the northwest, still seen in modern Dutch.

In what historical period did the different types of Germanic languages stop being collectively understood?
the Migration Period