viii THE ANIMAL KINGDOM. 



shown to possess such a structure. Furtlier researches may show the same to be true 

 of all forms and all cells. 



Genuine cells have a nucleus, the latter containing a nucleolus. It will thus be 

 seen that the true cell is not a simple body; the nucleus is distinct from the rest of 

 the cell in structure and appearance, and the nucleolus also differs in the same way 



©from the nucleus and rest of the cells. The nucleus and 

 nucleolus also vary in themselves in size and contents, the 

 "■' granules and fibres filling them varying in size and number. 

 i" This fact should lead us to regard the cell as not so simple as 

 I generally supposed. A recent writer, impressed by the com- 

 plexity of cell structure, has subjected to a critical examina- 

 tion the characters of ganglion cells, both smooth and striated 

 muscle-cells, glandular, liver, and salivary gland cells, and epi- 

 ^miokus^K«"nucieoius"' thelium, both that of the mucous membrane and that which is 

 ciliated, as well as that of the crystalline lens, beside cartilage 

 and embryonic cells. Both healthy and diseased cells are found by Arnold to jiossess 

 a complicated structure. The two constituents, as ordinarily distinguished, the cell 

 body and the cell nucleus, consist of a ground substance as well as of granules, sets of 

 granules, and filaments ; these latter may become very complicated in the more highly 

 developed forms of cells. Arnold would regard a cell as consisting of a nucleus and 

 of an investing mass, both of which contain, in a homogeneous ground substance, 

 granules and filaments. 



Dr. C. S. Minot believes that the weight of an animal depends on the number and 

 size of its cells, and that these two variables require to be determined before we can 

 speak definitely as to the processes of growth in animals. Minot points out that the 

 growth of a body is usually measured by its weight, but that this method takes no 

 account of the amount of non-protoplasmic matters present. All many-celled animals 

 "pass through successive cycles, in which we can distinguish the two processes of 

 senescence and of r^uvenation." As growth is a function of rejuvenescence effected 

 by impregnation, it follows that growth can only be measured by taking into account 

 the number of cells living at any given time. 



Animal cells are of microscopic size, but one-celled animals are in some cases large 

 enough to be detected with the naked eye ; such are many Foraminifera, and the Sten- 

 tor among Infusorians. Kolliker states that the size of the cells descends on the one 

 hand, as in many cells — the blood cells, etc. — to 0.002-0.0003 of a line, and attains 

 on the other, as in the cysts of the semen and the ganglionic globules, the size of 0.02 

 -0.04 of a line. The largest animal cells are certain gland-cells of insects which 

 measure up to 0.01 of a line, and the yolk-cells or ova, especially of birds and reptiles. 

 Animals grow by the self-division and multiplication of cells. This is the initial, 

 fundamental process which underlies reproduction and growth. The cells in any part 

 of the growing body divide into two, and these again sub-divide, the products of self- 

 division becoming as large as the original cell, and in this way the body increases in 

 size. 



The part of the cell in which this process of self-division begins is the nucleus. A 

 good deal has been written upon this subject. Some authors believe that in cell divi- 

 sion the nucleolus first divides, then the nucleus, and finally the cell. Prof. W. Flem- 

 ming, however, maintains that the nucleus first of all undergoes a change, " separating 

 into a network of highly refracting filaments, which take up coloring matters strongly, 



