Any attempt on my part by way of introduction or commendation of Professor Schwann's work, must, I feel, be altogether misplaced and unnecessary. The treatise has now been seven years before the public, has been most acutely investigated by those best competent to test its value, and the first physiologists of our day have judged the discoveries which it unfolds as worthy to be ranked amongst the most important steps by which the science of physiology has ever been advanced. The Royal Society of London has evinced its sense of the great merit of the work by awarding to its Author the Copley Medal for the year 1845. The extensive reputation and fully-acknowledged value of the original work, then, forbid my presuming that any one of my readers can be altogether unacquainted with it and the general outlines of the Cell-Theory; I may, however, I trust, be permitted to add a few words respecting the edition which is now presented to the Subscribers of the Sydenham Society.