178 SURVEY OF CELL-LIFE.
vesicle ere it occurs; the germinal vesicle, for example; in others, and this is the most common, the nucleus is still solid, and its development into a vesicle does not take place until a later period, or perhaps the change never occurs at all. When the cell is developed, the nucleus either remains stationary at its previous stage of development, or its growth proceeds, but not in proportion to the expansion of the cell, so that the intermediate space between it and the cell-membrane, the cell- cavity, is also constantly becoming relatively larger. If the growth of a cell is impeded by the neighbouring cells, or if the new molecules added between the existing particles of the cell-membrane are applied to the thickening of the cell-wall instead of to its expansion, it may occur that the nucleus becomes more vigorously expanded than the cell, and gradually fills a larger portion of the cell-cavity. An example of this was brought forward at page 23, from the branchial cartilages of the tadpole; on the whole, however, such instances are very rare. As the nuclei, in the course of their development, and especially in such instances as that just mentioned, continually lose their granulous contents and become pellucid, and as in some cases, the germinal vesicle for example, other corpuscles, such as fat-globules, &c., may be developed in these contents of the nucleus (a circumstance which never occurs with respect to the cell-cavities) it is often difficult to distinguish such enlarged nuclei from young cells. The presence of two nucleoli is often sufficient to enable us to distinguish such an enlarged hollow nucleus. The observation of the stages of transition, between the characteristic form of the cell-nucleus and these nuclei which so much resemble cells, will also aid us in obtaining the information desired. As in the case of the germinal vesicle, however, a positive decision can only be obtained by demonstrating that such a nucleus has precisely the same relation to the cell that an ordinary cell-nucleus has; that is to say, that such a nucleus is formed before the cell, that the latter is formed as a stratum around it, and that the nucleus is afterwards surrounded by the cell. Whether the nucleus undergoes any further development, as the expansion of the cell proceeds, or not, the usual result is that it becomes absorbed. This does not take place, however,
