THE ANIMAL CELL. 41 



filament becomes arranged in more or less distinct loops converging toward 

 the two poles, resembling somewhat in appearance a rosette or wreath (Fig. 6, c). 

 From the poles to the loops, fine threads, not staining like the others (achromatic), 

 are seen bridging across the space left between the filament and the cell- 

 protoplasm. These are known as the nuclear spindle. (4) The loops now- 

 become flattened so as to form a festooned ring or star at the equator 

 of the nucleus. This is known as the single star, aster, monaster. The loops 

 begin to break transversely at the equator (Fig. 6, D*), having sometimes 

 previously broken at their polar ends. The nuclear spindle or achromatin is very 

 distinct, as well as a radiating ari'angement of protoplasmic granules toward the 

 poles. It is at this stage, or sometimes after, that a longitudinal splitting of the 

 filaments occurs, so that they become more numerous and more slender. (5) After 

 breaking across at the equator, the chromatic filaments move toward the poles as if 

 they were guided by the achromatic threads. These threads bridge across between 

 the two receding stars, which are known as diaster or daughter stars. The pro- 

 toplasm, with its radiating granules, begins to group itself around the two poles 

 (Fig. 6, E). (6) The daughter stars have now reached the poles ; the broken ends 

 become united, so that each daughter chromatic filament becomes a single festooned 

 filament, forming a rosette or wreath, the daughter rosettes or wreaths. There 

 is now distinct evidence of cleavage in the protoplasm (Fig. 6, F). (7) By further 

 irregular contraction the regular arrangement of the loops becomes lost, and the 

 filament presents a convoluted appearance, constituting the daughter glomeruli or 

 skeins (Fig. 6, G). The cleavage of the protoplasm is now complete except where 

 the achromatic threads are found. (8) By further convolution and contraction 

 the loops of the filament become fused together, and form again a network. The 

 nuclear membrane which disappeared at the beginning of the karyokinesis is 

 formed anew, and two daughter cells with nuclei are formed (Fig. 6, H). The 

 remains of the achromatic threads bridge across the intercellular substance, but 

 later usually disappear completely. 



In the reproduction of cells by direct division the process is brought about 

 either by segmentation or by gemmation. In reproduction by segmentation or 

 n*$ion the nucleus first splits by becoming constricted in its centre, and thus 

 assuming an hour-glass shape. This leads to a cleavage or division of the whole 

 protoplasmic mass of the cell : and thus we find that two new cells have been 

 formed, consisting of the same substance as the original one, and each containing 

 a nucleus. These daughter cells are of course at first smaller than the original 

 mother cell : but they grow, and the process may be repeated in them, so that 

 multiplication may rapidly take place. In reproduction by gemmation a budding- 

 off or separation of a portion of the nucleus and parent-cell takes place, and, 

 becoming separated, forms a new organism. 



The cell-wall, which is not an essential constituent, and in fact is often absent, 

 consists of a flexible, transparent, structureless or finely striated membrane, which 

 is permeable to fluids. As far as is known, every animal cell is derived from a 

 pre-existing cell. The death of cells is accomplished either by their mechanical 

 detachment from the surface, preceded possibly by their bursting and discharg- 

 ing their contents, or by various forms of degeneration fatty, pigmentary, or 

 calcareous. 



EPITHELIUM. 



All the surfaces of the body the external surface of the skin, the internal 

 surface of the digestive, respiratory, and genito-urinary tracts, the closed serous 

 cavities, the inner coat of the vessels, and the ducts of all secreting and excreting 

 glands are covered by one or more layers of simple cells, called epithelium or 

 epithelial cells. These cells are also present in the sensory and terminal parts of 

 the organs of special sense, and in some other organs, as the pituitary and thyroid 

 bodies. They serve various purposes, forming in some cases a protective layer, in 

 others acting as an agent in secretion and excretion, and again in others being 

 concerned in the elaboration of the organs of special sense. Thus, in the skin, 



