250 



III 19J2 CoNKLiN ^) sliowed for diifeient species and individuals of 

 one species of Boat Shell (Crepidula), that in spite of the very great 

 differences in body size, "(he size of tissue cells is approximately 

 (he same in all species examined, and in all individuals of both 

 sexes and of very difïerent sizes, in the main, differences in body 

 size are due to differences in the number of cells present, and not 

 to variations in the size of individual cells. Ganglion cells and muscle 

 cells form the principal exception to this iiile". (According to his 

 measurements the diameter of muscle fibres is not greater in the 

 larger species, and only a little greater in large-sized individuals of 

 one species). From his measurements of a gigantic female and a 

 medium-sized male individual of Crepidula plana I find for the 

 exponent of relation of the volume of the body and the volume of 

 the ganglion cells the value of 0.3149, which is sufficierdly near 

 Vi8 to prove the existence of the same ontogenetic relation also in 

 the Invertebrates. 



As was already mentioned, Levi is less certain in his conclusion 

 about the muscle fibres; he generally finds them thicker in large 

 animals than in small ones, but (he thickness changes much less 

 than the length, and there are many exceptions to the rule. This 

 uncertainty is, indeed, explicable by what was derived above from 

 VON DEH Malsbuiig's measurements with regard to the larger differ- 

 ences between individuals than between the species. 



The nerve cells and the muscle cells are distinguished from 

 most other cells (only the fibres of the crystalline lens make an 

 exception to the general rule) in that early in life — in Man and all 

 Mammalia examined on this point about birth-time — they cease 

 increasing in number through division, but then continue for some 

 time to increase separately in volume. The other cells go on multi- 

 plying by division throughout life. The muscle cells continue increas- 

 ing their separate volume at least up to the adult state of the 

 individual. But the nerve cells also stop doing this in the early 

 youth of the individual. 



A consequence of this peculiarity of the nerve cells is, that 

 early in the life of the individual the brain assumes the volume of 

 the adult state of the body; in a male child for instance, at the 

 age of nine, in a female child when six years old. But a similar 

 remark holds among others for the Dog, the Rat, the Great Ant- 

 Eater, the Sparrow, the Chicken, the Crocodile, the Frog, the Salmon, 



1) Edwin G. Gonklin, Body Size and Cell Size. Journal of Morphology. Vol. 23, 

 p. 159—188. l^hiladelphia 1912. 



