Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/75

Rh  of several ova in one ovicapsule is difficult of explanation by Barry’s view. In the further investigation of this subject, attention must continue to be fixed upon the possible, and even probable, existence of a nucleus to the ovicapsule. Wagner saw certain follicles in the mole, in which he could not detect a trace of any enclosed body.

Wagner expresses himself in his new work (Lehrbuch der Physiologie, Leipzig, 1839, p. 34) as being doubtful whether the vesicles met with in his observations on the preformation of the germinal vesicle in the ova of insects, were actually vesicles or not. The observations of Barry on the ova of mammalia and birds, are, however, in favour of the explanation of the ovum of the insect originally given by the first-named highly respected investigator, and therefore also of that which represents the germ-vesicle as nucleus of the ovum-cell. It is true it might be said, that, regarding the germ-vesicle as a cell, a second one, the ovum-cell was formed around it; but as opposed to that view, it must be remembered that no example of a second cell being formed around the first is afforded amongst all the other cells which exhibit a nucleus of the decidedly characteristic form. The point in dispute, as to the interpretation to be placed upon the germ-vesicle, loses, however, somewhat of its importance if the theory which I shall propose (see the conclusion of the treatise) be received, inasmuch as I shall there endeavour to prove the formation of the cell around the nucleus to be merely a repetition of the process by which the nucleus is formed around the nucleolus, and that the whole process of development of the cell may be reduced to a single or many times repeated formation of strata. The germinal-vesicle accordingly is the first stratum, or a cell of the first order; the yelk-cell the second stratum, or a cell of the second order. As above stated at page 47, a minute point was observed in the germinal spot by Wagner, and subsequently by myself also; and my respected colleague Vanbeneden lately found germinal spots in the ova of certain polypes (Genus Zoanthus), and also in ova of Anodonta, which had not as yet left the ovary, that appeared granulous, but at the same time seemed to be hollow, and some of which distinctly contained a very small round corpuscle. This observation accords most completely with the theory which regards the cells as produced by a stratified formation. This small corpuscle, which may be called a secondary nucleolus, would here be the primordial formation; the germinal spot would be the first stratum around it, that having in this instance become developed into a vesicle, in a manner likewise to be explained hereafter by the Cell-Theory; the germinal vesicle would be the second, and the yelk-cell the third stratum. The formation of even a fourth stratum, the albumen membrane, around the yelk-cell, would involve nothing contradictory to the theory; but in such case we certainly could not avoid regarding it as a second cell, which had become formed around a previously existing one: for the yelk-cell cannot well be considered to be a nucleus. The mode of formation of this albumen membrane must, however, in the first instance, be ascertained by investigation. sequent relations of the vesicle seem also to afford evidence in its favour. The disc, for instance, is formed around it, and this perhaps corresponds to the granulous precipitate which