180 SURVEY OF CELL-LIFE. 



formation of the cell-contents frequently follows, giving rise to 

 a formation of new products in the cell-cavity. In most of 

 the hollow cell-nuclei, the contents become paler, less granu- 

 lous, and in some of them fat -globules, &c, are formed. (See 

 pages 173, 4.) We may therefore say that the formation of cells 

 is but a repetition around the nucleus of the same process by 

 which the nucleus was formed around the nucleolus, the only 

 difference being that the process is more intense and complete 

 in the formation of cells than in that of nuclei. 



According to the foregoing, then, the whole process of the 

 formation of a cell consists in this, that a small corpuscle (the 

 nucleolus) is the earliest formation, that a stratum (the nucleus) 

 is first deposited around it, and then subsequently a second 

 stratum (substance of the cell) around this again. The sepa- 

 rate strata grow by the reception of new molecules between 

 the existing ones, by intussusception, and we have here an illus- 

 tration of the law, in deference to which the deposition takes 

 place more vigorously in the external part of each stratum than 

 it does in the internal, and more vigorously in the entire ex- 

 ternal stratum than in the internal. In obedience to this law 

 it often happens that only the external part of each stratum 

 becomes condensed into a membrane (membrane of the nucleus 

 and membrane of the cell), and the external stratum becomes 

 more perfectly developed to form a cell, than the nucleus does. 

 When the nucleoli are hollow, which, according to Schleiden, 

 is the case in some instances in plants, perhaps a threefold 

 process of the kind takes place, so that the cell-membrane 

 forms the third, the nucleus the second, and the nucleolus the 

 first stratum. Probably merely a single stratum is formed 

 around an immeasurably small corpuscle in the case of those 

 cells which have no nuclei. 



Varieties in the development of the cells in different tissues. 

 Although, as we have just seen, the formative process of the 

 cells is essentially the same throughout, and dependent upon a 

 formation of one or many strata, and upon a growth of those 

 strata by intussusception, the changes, on the other hand, which 

 the cells, when once formed, undergo in the different tissues, 

 are, in their phenomena at least, much more varied. They 

 may be arranged in two classes according as the individuality 



