From its property of originating the membrane of the cell, Schleiden gave to the nucleus the name of Cytoblast or cellgerm. In the cytoblast he saw what seemed a smaller body, the nucleolus; and he, as well as Schwann, Müller, Valentin, and Henle, supposed the smaller body to exist first,—the substance of the cytoblast, or larger body, they believed to be deposited around the smaller one, and the substance of the membrane of the cell to be deposited around the cytoblast.* Fig. 1. According to my observations, it is not so: I find the process to be of a widely different character; so different, indeed, that it may be termed the very opposite of that supposed by the German physiologists just mentioned. It is true that a minute pellucid, and, as will presently be seen, a very important globule (fig. 1), exists before the cytoblast. But it is not true that a finely granular substance is "deposited around" that pellucid globule, so as to make the nucleolus seen in the cytoblast identical with the previously existing pellucid globule. That previously existing pellucid globule absorbs and assimilates new matter, and, at the same time, enlarging and becoming finely granular, assumes the form in fig. 4, the cytoblast of Schleiden,—having prepared a nucleolus for itself. Nor is it true that the membrane of the cell is formed around the cytoblast. The outer part of the cytoblast rises in the form of membrane, so as to produce fig. 5, a cell; another portion of the cytoblast enters into the formation of the contents of the cell; and what is left of the cytoblast in the cell-wall, becomes the nucleus of the cell. This nucleus, said to remain inert in the cell-wall, or to *The following is the substance of a statement made by Valentin, as briefly expressing the views on this subject of Schleiden, Schwann, Müller, Henle, and himself; and as being descriptive of the first formation of the elements of tissues—In a fluid, says he, there are precipitated granules, which are nucleoli; around the nucleolus there is deposited a finely granular substance, by which there is formed the nucleus (cytoblast); and around the nucleus there is formed the membrane of the cell. The principle of formation of the nucleus around the nucleolus, is essentially the same as that of the cell around the nucleus. Valentin concludes that this process may be described by the ex pression, heterogeneous circum-position.-Repertorium, 1839, ii., p. 284. be "cast off as a useless member" and " absorbed," is the seat of a most elaborate process, which I will now endeavour to describe first remarking, that the woodcuts are intended for the purpose of assisting in the description of that process, rather than as minutely representing nature. Drawings in which the latter has been attempted, taken from hundreds in my memoirs, are given in the plates; and these will be ren dered more intelligible by the woodcuts. (In many of the drawings, for instance in those of fig. 39, Plate I., the structure is so minute that it should be viewed with a pocket lens.) " The cytoblast" of Schleiden has usually a discoid or amygdaloidal form, and in substance appears finely granular* (fig. 4); and his "nucleolus," the pellucid point in fig. 4, I find to represent the situation of a brilliantly pellucid substance, which, from its appearance, I have called hyaline.† Fig. 2. Two portions of hyaline enlarged, and surrounded by globules of extreme minuteness, which seem to have their origin in the bodies they surround. One of these bodies presents a faint circular marking at the side, not yet formed in α Fig. 2. the other. This circular marking is the incipient nucleolus. Compare with a and ẞ, the two states of nuclei, in Fig. 33, Plate I.; a nucleolus being there present in ß, and not yet formed in a. Fig. 3. That which was a large portion of hyaline in fig. 2, is now an incipient cytoblast, and already Fig. 3. membranous at the surface; the minute globules, now no longer seen, having apparently entered into the formation of its membrane. Compare with many figures in Plate I. Fig. 4. Fig. 4. The cytoblast of Schleiden; differing from the body fig. 3, in being finely granular, except in the region of the circular marking or nucleolus, which first made its appearance in one of the portions of hyaline fig. 2. Compare with figs. 35, 36, &c., Plate I. * In animals this finely granular substance is, or corresponds to, the red colouring matter of the corpuscles of the blood. See figs. 35, 36, &c., in Plate I. † A term suggested to me by Professor Owen. Fig. 5. Fig. 5. The cytoblast has become a cell; its contents finely granular,-the remains as they are of the finely granular substance of the cytoblast in the preceding figure. That which was the nucleolus in figs. 2, 3, 4, is now the nucleus. This nucleus, a mass of hyaline, is surrounded by minute globules, * as in fig. 2; and, like one of the bodies in that figure, presents, in its turn, an ill-defined nucleolus. Compare with two of the cells ẞ, in fig. 34 of Plate I. They Fig 6. Fig. 6. The minute globules are no longer seen. appear to have entered into the formation of the membrane by which the hyaline nucleus is now surrounded. This hyaline nucleus is in the same state as the entire body fig. 3,—that of a young cytoblast. It represents the "hollow nucleus" and "inner cell" of authors. The nucleolus is now well-defined. Compare with y of fig. 34, in Plate I; and read the description of that figure. Fig. 7. Fig 7. The hyaline nucleus of the cell has become finely granular, and is now as much a cytoblast as the body, fig. 4. Compare with the nucleus of several cells in fig. 39 of Plate I., and with the concentric cytoblasts in fig. 41 of the same Plate. Fig. 8. The nucleus or cytoblast of fig. 7 has become a cell, and that which was the nucleolus in fig. 7, is now the nucleus of this cell. Farther, in the centre of the nucleus which had been a nucleolus, another nucleolus is coming into view. The hyaline, much increased in quantity, is giving off globules into the surrounding substance, and is hence nodular or star-like in its form. cells on the right hand, and with other Plate I. Fig. 8. Compare with two bodies in fig. 39 of Fig. 9. The segments of the nodular or star-like hyaline *The first contents of all cells have a like mode of origin; and it is not easy to say where the cytoblast terminates, and the cell begins; so that often one is obliged to use almost indiscriminately the terms "cytoblast" and "cell." Fig. 9. have assumed independent forms; having appropriated the finely granular substance into which they were cast, and thus become young cytoblasts. What was left of the central hyaline has increased in quantity, and, as in figs. 2 and 5, has become surrounded with minute globules. Compare with blood-corpuscles above the middle part of fig. 39, in Plate I. Fig. 10. The young cytoblasts are larger. globules are no longer seen, having probably entered into the formation of the membrane by which the central hyaline is now surrounded. This central hyaline is in the same state as the nucleus of the cell in fig. 6. It is a young cytoblast. Compare with several cells in fig. 39, of Plate I. The minute Fig. 10. Fig. 11. What were young cytoblasts around the central hyaline, are now young cells; which sur round what is now a true cytoblast; for the central hyaline, pellucid in fig. 10, is now finely granular. Compare with ß in fig. 37, and with several cells in fig. 39 of Plate I. Fig. 11. Figs. 12, 13, 14, 15. To describe the central portion of the nucleus in these figures separately, would be to repeat the description just given of the entire nucleus in figs. 8, 9, 10, 11; for the appearances are the same, and referable to a continuation of the same process. Fig. 12. Fig. 13. Fig. 14. Fig. 15. The only remarks to make concerning figs. 12 to 15, are se, viz., that the layer of young cells already formed in . 11, becomes pushed farther and farther into the interior f the cell by the succeeding brood; and that as each nucleus is transformed into a layer or brood of cells, its place is taken by the enlarging nucleolus; which again is succeeded by a fresh nucleolus, arising in the centre from which the former had enlarged. Compare with a cell in the woodcut fig. 26, and with a corresponding cell in fig. 39 of Plate I. And this process, it will be seen, begins before the complete formation of even Schleiden's cytoblast. See figs. 2, 3. The membrane of the nucleus or cytoblast, becoming the inner cell in fig. 8, and represented in all the succeeding figures, though distinct enough, capacious, and for some important purpose filled with pellucid fluid when in a central situation, (Plate I., fig. 45 ß),—is rarely seen with distinctness when subsequently pushed out from the centre by a fresh brood, and made to separate two layers of cells: and often it entirely disappears, as shewn by several bodies in fig. 39, Plate I. I have just spoken of the cell-membrane, as formed by minute globules. An immeasurably minute cell-formation, however, seems even here to intervene. Fig. 38 in the Plate presents part of the membrane of a very important cell produced by the coalescence of minuter cells, the formation of which it is not difficult to follow.* And you afterwards find in its substance the nuclei, or germinal centres, of the cells which formed it (see the figure). Yet so very small is that cell at first, that two hundred millions of them could be contained in a cubic inch. To that cell (my ovisact) I shall presently return. I wish now to add, that there is thus a considerable difference between my views and those of the German physiologists just mentioned, as to the mode of origin not only of the cytoblast, but of the membrane of the cell. The substance of the larger body is not deposited around the the smaller, but the smaller is transformed into the larger. The nucleolus becomes a cytoblast, and the cytoblast becomes a nucleated cell. From the foregoing remarks and figures it will also be *See in Phil. Trans., 1841, Plate xxv., figs. 170, 171, 172, 173. |