CELL FORMATIOX. 349 



come detached, forming in their turn cells ; this is also termed exogenous 

 generation, inasmuch as the process takes place from the exterior. 

 Endogenous generation is the second mode, and by it is meant that 

 one cell is formed within the body, as it were, of the parent cell. 

 The third manner is denominated fissiparous generation, and is where 

 one cell becomes constricted, and eventually, at the point of constric- 

 tion, divides into two. Of these three kinds or modes of multiplica- 

 tion, one only is found to occur in animals — it is the endogenous. The 

 exogenous is only seen in the lowest plants. Fissiparous generation 

 occurs iu vegetables, and has been supposed by some to occur in 

 animals also. The most striking example of endogenous generation 

 that can be adduced, is that which takes place in the ovum of animals 

 or birds. The first part formed of the ovum is the germinal cell, in 

 the centre of the yolk ; this approaches the surface, is dissolved, and 

 there is then developed a new cell, which is called the embryo cell, 

 from which is generated a numerous progeny ; the contends of these 

 cells are the cytoblastema to other nucleated cells. 



As an instance where cells are not directly derived from cells, but 

 previously-existing cells exert an influence ,on those to be formed, 

 we may instance a fractured bone, between the ends of which osseous 

 matter is deposited. We infer from this, that the substance of the 

 bone determines, as it were, the formation of other cells, first into 

 cartilage, and then into bone. This change is shown at figs. 8 and 9, 

 Plate XII. Generally, however, where a part has to be repaired, it 

 does not seem to determine the generation of a texture similar to 

 itself — muscle and skin, for examples. We have an exception to the 

 last observation in the case of nerves, which, if cut across, a substance 

 is formed between the ends which can transmit the nervous influence ; 

 but the ends must not be separated to any great distance, or this will 

 not occur. The same remark applies to bone. Cells may retain an 

 independent existence, although changes may take place in their walls 

 and contents, or they may become eventually dissolved and be succeeded 

 by new ones. They may change their form, that is, a globular cell may 

 pass to the compressed form ; and this may arise from the difference 

 of the contents to the material outside it, as iu the corpuscles of the 

 blood. Flattening of cells may arise from the pressure they exert 

 upon each other, as, for example, in the cells of the epidermis and the 

 epithelium. In some cases the cells become so thin, that their thick- 

 ness cannot be measured. Sometimes, where there is but a single 

 layer of cells, flattened, the hexagonal form of cell is assumed ; at 

 other times the polygonal. When a mass of cells compress each other, 



