176 SURVEY OF CELL-LIFE
molecules between the particles of this membrane takes place in such a manner that more molecules are deposited between those particles which he side by side upon its surface than there are between those which lie one beneath another in its thickness, the expansion of the membrane must proceed more vigorously than its increase in thickness, and therefore a constantly increasing space must be formed between it and the nucleolus, whereby the latter remains adherent to one side of its internal surface.
I have made no observations on the formation of nuclei with more than one nucleolus. But it is easy to comprehend how it may occur, if we conceive that two nucleoli may lie so close together that the layers which form around them become united before they are defined externally, and that by the progressive deposition of new molecules, the external limitation is so effected that two corpuscles are enclosed by it at the same time, and then the development proceeds as though only one nucleolus were present.
When the nucleus has reached a certain stage of development, the cell is formed around it. The following appears to be the process by which this takes place. A stratum of substance, which differs from the cytoblastema, is deposited upon the exterior of the nucleus. (See pl. III, fig. 1, d.) In the first instance this stratum is not sharply defined externally, but becomes so in consequence of the progressive deposition of new molecules. The stratum is more or less thick, some- times homogeneous, sometimes granulous; the latter is most frequently the case in the thick strata which occur in the formation of the majority of animal cells. We cannot at this period distinguish a cell-cavity and cell-wall. The deposition of new molecules between those already existing proceeds, however, and is so effected that when the stratum is thin, the entire layer—and when it is thick, only the external portion—he- comes gradually consolidated into a membrane. The external portion of the layer may begin to become consolidated soon after it is defined on the outside; but, generally, the membrane does not become perceptible until a later period, when it is thicker and more defined internally ; many cells, however, do not exhibit any appearance of the formation of a cell-membrane, but they seem to be solid, and all that can be remarked
