247 



their measurements the muscle fibres in Ihe gastrocnemius of a 

 mouse about as thick as in tlje homologous muscle of a woman 

 and of a dog (but thinner than in that of a man). In the masseter 

 of their mouse the muscle fibres were about as thick as in the 

 masseter of the man, but less thick than in the dog. That the size 

 of the body from species to species has only little influence on the 

 caliber of the muscle fibres appears also from this that G. Levi ') 

 found the diameter of the thickest muscle tibres in the rectus femoris 

 of a mouse not below that in a lat (twenty times as heavy). 



It may, therefore, be admitted that in homoneuric species the 

 number of muscle fibres, and then also proportionally that of the 

 ganglion cells in the bi-ain, greatly increases with the size of the body. 

 But the available data do not enable us to calculate the exact 

 relation of Ihe body weight to the number of the muscle fibres 

 in these species. On the ground, however, on one side of the 

 relation found for the brain weight E to the body weight P, 

 according to which E increases propoitionally to P^l^ between 

 homoneuric species, and proportionally to P^/i» between adult indi- 

 viduals of the same species, and on the other side, on the gi-ound 

 of the relation found for the volume of the separate nerve 

 cells C to the body weight, according to which C increases in 

 the ratio of P^l^», both between individuals and between species; 

 further on the gi-ound of the established fact that between large 

 and small individuals the number of the muscle fibres, hence 

 proportionally that of the nerve cells in the brain, does not differ, 

 but that it differs greatly between large and small species, we may 

 conclude, that also the number of the nerve cells between 

 homoneuric species increases in the ratio of PVis. 



The difference l)et ween tlie [> hy 1 ogen e t ic and the ontogenetic 

 exponent is thus rationally explained. It means that in the origina- 

 tion of the species, increase of the size of the body is accompanied 

 with multiplicaiion of the nerve cells, through cell division (in non- 

 homoneuric species this multiplication is gieater in certain parts of 

 the brain)*). With the establishment of larger adult individuals 



1) Giuseppe Levi, Studt sulla grandezza delle cellule. Archivio di Anatomia e 

 di Embriologia. Vol. V, p 327. Firenze 1906. 



3) Direct counting of the cells in the grey cortex of Monkeys by Otto Mayer 

 (Mikrometrische Untersuchungen iiber die Zeildichligkeit der Grosshirnrinde bei 

 den Affen, Journal fur Psychologie und Neurologie, Bd. 19, p. 237. Leipzig 1912) 

 teaches that per m.m.', calculated throughout the cortex, only about the same 

 number of cells occur in the small Hapale (8448) as in the larger Ghrysothrix 

 (3603) and in the still larger Cebus (3581). As the brain weights in these hetero- 

 neuric and from the smallest to the largest species higher cephalized American 



J 6* 



