It is an opinion now very generally entertained by anatomists that in vertebrate and in certain invertebrate animals, there are in connexion with the nervous system apolar, unipolar, and multipolar, including bipolar cells. It is easy to demonstrate the presence of multipolar cells, but it is another matter altogether to prove that certain cells which seem to be apolar or unipolar are really of this nature. An observer is justified in asserting very positively the existence of that which he has

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... himself distinctly seen and has shown to others; but it does not follow that he is correct in concluding that, because a fibre or other structure has not been seen by him in certain specimens, it therefore does not exist, for the actual exist ence of a structure and its demonstration are two very different questions in minute anatomical inquiry. The one is a question of fact, no matter how it may be explained, or how many different interpretations may be offered of it. The other is but an infer ence arrived at in the absence of evidence, and may result from imperfect means of observation, or want of due care on the part of the observer in preparing the specimen. A new fact, if capable of being demonstrated to others, will be received at once as true, but negative assertions, although supported by the authority of the most expe rienced, ought in matters of observation to be received with the greatest caution. It is one thing to assert that a cell has no fibre proceeding from it, or that a cell has but one fibre, and another to state that no fibre or but one fibre has been demonstrated. Although many general facts are opposed to the notion of the presence of apolar and unipolar cells, the existence of such cells is generally admitted and taught. If apolar and unipolar cells exist at all, they are certainly less numerous than the multipolar cells; and it is clear that the arrangement and action of the three classes of cells, , unipolar, and m u l t i p o l a r, must be very different. Indeed it almost follows, if the three classes of cells do exist, that there must be as many distinct kinds of principles of action. Nerve-cells, which bear such different relations to nerve-fibres as must exist in the case of the cell unconnected with any fibre at all, the cell connected with one, and that connected with more than one fibre, cannot possibly influence the fibres in precisely the same manner. The supposition of such an anatomical arrangement involves the exist ence of different principles of action. m d c c c l x i i i . 4 E on July 20, 2018 http://rstl.royalsocietypublishing.org/ Downloaded from on July 20, 2018 http://rstl.royalsocietypublishing.org/ Downloaded from APOLAR, UNIPOLAR, AND BIPOLAR NERYE-CELLS OE THE EROGL 545 * This question has been fully considered in my lecture " On the Structure and Growth of the Simple Tissues," delivered at the Royal College of Physicians in 1860; and the general conclusions then arrived at, and since confirmed by other observations, w ill he found to be strongly supported by facts recounted in this paper. on July 20, 2018 http://rstl.royalsocietypublishing.org/ Downloaded from * My friend Mr. L ockhart Clarke has not observed this division of the cells which enter into the formation of the ganglia on the posterior roots of the nerves of mammalia. He describes the gradual increase in size and the alteration in structure and form of the cells as development advances, hut says nothing about their increase in number. I t is true that Mr. Clarke's observations were made upon mammalia, while the statements in my paper refer only to the frog; hut it is almost certain-I think indeed that it is quite certain-that if the cells do not multiply by division in mammalia they do not do so in the frog. Upon this question of fact, therefore, I regret to say that Mr. Clarke and myself are at issue.