SURVEY OF CELL-LIFE. 175
formed, and around that the germinal vesicle, which is the nucleus of the ovum-cell, Eizelle. [1] The youngest germinal vesicle there represented by Wagner, appears to be hollow. This is not generally the case, however, in the formation of cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nucleus of a cartilage-cell in the act of forming. A small round corpuscle is there seen, surrounded by some minutely granulous substance, whilst the rest of the cytoblastema is homogeneous. This granulous substance is gradually lost around the object; at a subsequent period it begins to be sharply defined, and then exhibits the form of a cell- nucleus, which continues to grow for a certain period. (See pl. III, fig. 1, a, b.) Such a nucleus usually appears solid in the first instance, and many nuclei remain in this condition; in others, on the contrary, the portion of the substance situated nearest to the external surface continually becomes darker, and not unfrequently at last forms a distinctly perceptible membrane, so that the nucleus is hollow in such instances. The formative process of the nucleus may, accordingly, be conceived to be as follows: A nucleolus is first formed; around this a stratum of substance is deposited, which is usually minutely granulous, but not as yet sharply defined on the outside. As new molecules are constantly being deposited in this stratum between those already present, and as this takes place within a precise distance of the nucleolus only, the stratum becomes defined externally, and a cell-nucleus having a more or less sharp con- tour is formed. The nucleus grows by a continuous deposition of new molecules between those already existing, that is, by intussusception. If this go on equably throughout the entire thickness of the stratum, the nucleus may remain solid ; but if it go on more vigorously in the external part, the latter will become more dense, and may become hardened into a membrane, and such are the hollow nuclei. The circumstance of the layer generally becoming more dense on its exterior, may be explained by the fact that the nutritive fluid is conveyed to it from the outside, and is therefore more concentrated in that situation. Now if the deposition of the new
- ↑ See the Supplement.
