Sentences of a paragraph are given below in jumbled order. Arrange the sentences in the right order to form a meaningful and coherent paragraph.
A. According to his hypothesis, now known as the neuron theory, each nerve cell communicates with others through contiguity rather than continuity.
B. The watershed of all studies of the nervous system was an observation made in 1889 by Spanish scientist Santiago Ramón y Cajal.
C. It has since been proved that Cajal’s theory is not universally true, but his central idea has remained an accurate guiding principle for all further study.
D. That is, communication between adjacent but separate cells must take place across the space and barriers separating them.