Page:Popular Science Monthly Volume 83.djvu/189

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of the œsophagus, 6 pairs of cells in the excretory tubules, and 13 cells in the cingulum, one of which is on the dorsal mid line.

In the nematode Ascaris megalocephala Goldschmidt found 162 cells in the nervous system, while Martini finds 65 muscle cells in Oxyuris, and 87 muscle cells in Sclerostoma, the latter being derived from 65 cells of an earlier stage.

A similar constancy of cell number has been found by Woltereck in Polygordius larvæ, by Apathy in the central nervous system of Hirudinea, by Gaule and Donaldson in spinal ganglia of frogs, and by many investigators in small but highly differentiated parts, such as the ommatidia of compound eyes, the lens fibers of vertebrate eyes, the nurse cells of certain arthropod and annelid ova, etc. Such cases of cell constancy are, as Martini remarks, "the crowning fact of determinate development." In all such cases the definite number of cells in the entire body or in a particular organ must be determined by a definite number of cell divisions which proceed from the egg, or from the protoblast of the organ, and this limitation in the number of cell divisions must in some way be determined by heredity. Since increase of differentiation is associated with decrease of cell division, the latter being stopped altogether when differentiation has reached a certain stage, it seems probable that all cases of cell constancy are due to constancy of differentiation.

Where the number of cells in an organ or in an animal is very large it is not possible to prove that the cell number is constant, but in many cases where cell division ceases in embryonic stages the cell number is constant. In such cases cell division does not continue after differentiation is complete, though cell growth does. To all such cases in which there is cell constancy Martini gives the name "Eutelie."

On the other hand, there are many animals in which the number of cells in any particular organ is not constant but is proportional to the size of the organ. In Crepidula the number of egg cells within the ovary and the number laid in any season varies with the size of the animal, but the size of individual eggs remains constant for each species; the same is also true of epithelial cells, gland cells and blood cells. The divisions by which such cells are formed are in general non-differential, and since both growth and division in such cases continue throughout life the size of any given type of cell is fairly uniform whatever the body size may be. In differential cell divisions, or in highly differentiated