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174 SURVEY OF CELL-LIFE. lages of the tadpole, or larger corpuscles may be subsequently formed in the interior of hollow nuclei, for instance, the innumerable corpuscles in the germinal vesicle of the fish, and fat-globules in the nucleus of the fat-cells in the cranial cavity of fishes.

The nucleus, in most instances, contains one or two, more rarely three or four small dark corpuscles, the nucleoli. Their size varies from that of a spot which is scarcely discernible to that of Wagner’s spot (macula germinativa) in the germinal vesicle. Nucleoli cannot be distinctly recognized in all cell-nuclei. They may be distinguished from the larger corpuscles, which are sometimes developed in certain hollow nuclei, from the circumstance of their being formed at a much earlier period; they exist, indeed, before the cell-nucleus. They are placed eccentrically in the round nuclei, and in the hollow ones are distinctly seen to lie upon the internal surface of the wall. It is very difficult to ascertain their nature; it may also vary very much in different cells, They sometimes appear to be capable of considerable enlargement, as in the nuclei of the fat-cells in the cranial cavity of the fish, and in such instances often have the appearance of fat. According to Schleiden, hollow nucleoli also frequently occur in plants.

Most cell-nuclei agree in the peculiarity of not being dissolved, or rendered transparent by acetic acid, at least not rapidly so, whilst the cell-membrane of animal cells is in most cases very sensitive to its action. Some cells, (such as those of the yelk-cavity of the egg, plate II, fig. 3,) which have no perceptible nucleus of the ordinary form, exhibit a globule having the appearance of a fat-globule, which grows as the cell expands, though not in the same proportion, and was probably formed previous to the cell. Whether such a globule have the signification of a nucleus or not, must re- main an undecided question.

The formation of the cell-nucleus. In plants, according to Schleiden, the nucleolus is first formed, and the nucleus around it. The same appears to be the case in animals. According to the observations of R. Wagner on the development of ovain the ovary of Agrion virgo, the germinal spot is first