Sentence:  There is no doubt, however, that if endarchy is primitive in the protostelic and immediately derived types, it very rapidly gives place to mesarchy and <WSD>exarchy</WSD> in response to various demands, and it is not difficult to show how this may have occurred. 
Meaning of the word marked with <WSD> tags:  A vascular system in which development starts in the regions farthest from the axis and spreads inward.