THE SIZE OF ORGANISMS 185 



of the oesophagus, 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 9 found 162 

 cells in the nervous system, while Martini 6 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 10 

 in Polygordius larvae, by Apathy in the central nervous system of 

 Hirudinea, by Gaule and Donaldson 11 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 proto- 

 blast of the organ, and this limitation in the number of cell divisions 

 must in some way be determined by heredity. Since increase of differ- 

 entiation 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 differentia- 

 tion 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 



8 Goldschmidt, "Das Nervensystem von Ascaris, etc.," Zeit. wiss. Zool., 90, 

 1908. 



10 Woltereck, "Beitrage zur praktischen Analyse der Polygordiusentwick- 

 lung," Arch. Entw. Mech., 18, 1904. 



"Donaldson, "The Growth of the Brain," Scribners, New York, 1895. 



VOL. LXXXIII. — 13. 



