Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/29

INTRODUCTION. 3 latter in reference to animal organization, I here introduce an abridgment of Schleiden’s description of it. A delineation is given in plate I, fig. 1, a, a, taken from the onion. This structure — named by R. Brown, Areola or cell-nucleus, by Schleiden, Cytoblast — varies in its outline between oval and circular, according as the solid which it forms passes from the lenticular into the perfectly spheroidal figure. Its colour is mostly yellowish, sometimes, however, passing into an almost silvery white; and in consequence of its transparency, often scarcely distinguishable. It is coloured by iodine, according to its various modifications, from a pale yellow to the darkest brown. Its size varies considerably, according to its age, and according to the plants, and the different parts of a plant in which it is found, from 0:0001 to 0:0022 Paris inch. Its internal structure is granular, without, however, the granules, of which it consists, being very clearly distinct from each other. Its consistence is very variable, from such a degree of softness as that it almost dissolves in water, to a firmness which bears a considerable pressure of the compressorium without alteration of form. In addition to these peculiarities of the cytoblast, already made known by Brown and Meyen, Schleiden has discovered in its interior a small corpuscle (see plate I, fig. 1, 4,) which, in the fully-developed cytoblast, looks like a thick ring, or a thick-walled hollow globule. It appears, however, to present a different appearance in different cytoblasts. Sometimes only the external sharply-defined circle of this ring can be distinguished, with a dark point in the centre,—occasionally, and indeed most frequently, only a sharply circumscribed spot. In other instances this spot is very small, and sometimes cannot be recognized at all. As it will frequently be necessary to speak of this body in the following treatise, I will for brevity’s sake name it the “nucleolus,” (Kernkorperchen, ‘nucleus-corpuscle.”) According to Schleiden, sometimes two, more rarely three, or, as he has personally informed me, even four such nucleoli occur in the cytoblast. Their size is very various, ranging from the semi-diameter of the cytoblast to the most minute point. The following is Schleiden’s description of the origin of the cells from the cytoblast. So soon as the cytoblasts have attained their full size, a delicate transparent vesicle, the young cell, rises upon their surface, and is placed upon the flat cytoblast