He then goes on to define his view of the nucleus (cytoblast) and of the development of the cell around it, saying: "As soon as the cytoblasts have attained their full size, a delicate transparent vesicle arises upon their surface. This is the young cell." As to the position of the nucleus in the fully developed cell, he is very explicit: "It is evident," he says, "from the foregoing that the cytoblast can never lie free in the interior of the cell, but is always enclosed in the cell-wall," etc. Schleiden fastened these errors upon the cell-theory, since Schwann relied upon his observations. On another point of prime importance Schleiden was wrong: he regarded all new cell-formation as the formation of "cells within cells," as distinguished from cell-division, as we now know it to take place. Schleiden made no attempt to elaborate his views into a comprehensive cell-theory, and therefore his connection as a co-founder of this great generalization is chiefly in paving the way and giving the suggestion to Schwann, which enabled the latter to establish the theory. Schleiden's paper occupies some thirty-two pages, and is illustrated by two plates. He was thirty-four years old when this paper was published, and directly afterward was called to the post of adjunct professor of botany in the University of Jena, a position which with promotion to the full professorship he occupied for twenty-three years. Schwann's Treatise.--In 1838, Schwann also announced his cell-theory in a concise form in a German scientific periodical, and, later, to the Paris Academy of Sciences; but it was not till 1839 that the fully illustrated account was published. This treatise with the cumbersome title, "Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants" (_Mikroscopische Untersuchungen über die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen_) takes rank as one of the great classics in biology. It fills 215 octavo pages, and is illustrated with four plates. "The purpose of his researches was to prove the identity of structure, as shown by their development, between animals and plants." This is done by direct comparisons of the elementary parts in the two kingdoms of organic nature. His writing in the "Microscopical Researches" is clear and philosophical, and is divided into three sections, in the first two of which he confines himself strictly to descriptions of observations, and in the third part of which he enters upon a philosophical discussion of the significance of the observations. He comes to the conclusion that "the elementary parts of all tissues are formed of cells in an analogous, though very diversified manner, so that it may be asserted that there is one universal principle of development for the elementary parts of organisms, however different, and that this principle is the formation of cells."