Sentence:  There is no doubt, however, that if <WSD>endarchy</WSD> is primitive in the protostelic and immediately derived types, it very rapidly gives place to mesarchy and exarchy 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 nearest the axis center and spreads outward.